Midnight Navy Diamond Lattice Mermaid Gown: Free Crochet Pattern

There is something undeniably magical about a mermaid gown. The way it hugs the body, flows at the hem, and moves with every step creates a silhouette that feels both timeless and otherworldly. Now imagine that gown worked in a rich midnight navy yarn, covered in an intricate diamond lattice stitch that catches the light like scales beneath the sea. That is exactly what this free crochet pattern delivers — a stunning, wearable piece of fiber art that will challenge your skills, reward your patience, and leave everyone in the room speechless.

Whether you are crocheting this gown for a special occasion, a cosplay event, a bridal shower, a styled photo shoot, or simply because you deserve to feel extraordinary on a random Tuesday, this pattern is designed to take you from first foundation chain to finished hem with clear, detailed guidance. Grab your hook, your favorite skein of midnight navy yarn, and let’s make something breathtaking together.

About This Pattern

This Midnight Navy Diamond Lattice Mermaid Gown is an intermediate-to-advanced level crochet pattern. It is worked in the round from the bodice down, with the diamond lattice motif repeating through the waist and hip sections before the skirt flares dramatically at the knee line into a classic mermaid silhouette. The lattice creates a beautiful open-work texture that adds elegance and dimension without sacrificing structure, and the midnight navy colorway gives the finished garment a deep, sophisticated tone that reads as formal and polished.

The pattern is written for a standard size medium but includes notes throughout for adjusting to other sizes. Because garments are highly personal, always take your measurements before beginning and swatch generously. A good swatch is the difference between a gown that fits beautifully and one that ends up as a very fancy wall hanging.

Materials You Will Need

To create this gown, gather the following supplies before you begin.

For yarn, you will need approximately 1,800 to 2,200 yards of a sport or DK weight yarn in midnight navy. The exact yardage depends on your size and gauge. A yarn with some sheen works beautifully for this project — options like cotton-blend yarns, bamboo-blend yarns, or mercerized cotton all capture the light in a way that elevates the lattice stitch. Avoid anything too fuzzy or bulky, as the diamond lattice relies on clean stitch definition to show off its structure.

For your hook, a 3.5mm or 4.0mm crochet hook is recommended, but again, always defer to your gauge swatch. The hook size is a starting point, not a rule.

You will also need a set of stitch markers, a tapestry needle for weaving in ends, measuring tape, scissors, and approximately two yards of matching ribbon or elastic for the waist casing if you choose to add it. Optional but highly recommended: a dress form or patient friend to try the gown on as you work.

Gauge

Before you do anything else, please make a gauge swatch. Crochet a 6-inch square using the diamond lattice stitch pattern described below and measure carefully after blocking. Your goal is 20 stitches and 22 rows per 4 inches. If your swatch is smaller than this, go up a hook size. If it is larger, go down a hook size. Adjust until your gauge matches, then begin the gown.

Blocking your swatch is not optional for this project. The open-work nature of the lattice means blocking dramatically affects the final dimensions of your work. Use your preferred wet blocking or steam blocking method and allow the swatch to dry completely before measuring.

Stitch Glossary

Throughout this pattern, the following abbreviations are used.

ch means chain. sl st means slip stitch. sc means single crochet. hdc means half double crochet. dc means double crochet. tr means treble crochet. sk means skip. sp means space. rep means repeat. pm means place marker. rm means remove marker. RS means right side. WS means wrong side. beg means beginning. rem means remaining.

The diamond lattice stitch is the signature element of this gown. To understand it before diving into the full pattern, here is a simplified explanation. The lattice is formed by working chains and double crochets in a repeating diagonal sequence that creates open diamond shapes across the fabric. Every two rows, the diamonds complete themselves and a new set begins offset from the previous, creating that interlocking geometric grid. Once you work through the first full repeat, the rhythm becomes intuitive and you will find yourself flying through it.

The Diamond Lattice Stitch (Detailed)

To practice the diamond lattice stitch independently before applying it to the gown, work a foundation chain in a multiple of 8 plus 4.

Row 1 (RS): Dc in 4th ch from hook, ch 2, sk 2 ch, dc in next ch, rep across, ending with dc in last ch. Turn.

Row 2 (WS): Ch 4, turn. Sk first dc, dc in next dc, ch 2, dc in next dc across. End with dc in top of turning chain. Turn.

Row 3 (RS): Ch 5, turn. Sk first dc, dc in next dc, ch 2, dc in next dc. Continue across, working a ch 2 between each dc. Turn.

Row 4: Begin the diagonal crossing. Ch 3, dc in 3rd ch-2 sp, ch 2, dc in same sp, ch 2. Continue working into alternating chain spaces across to create the diagonal arms of the diamond. Turn.

Rows 5 through 8 complete the diamond by bringing the diagonal lines back together. By row 8, you will have a full row of closed diamond shapes. Rows 9 onward begin the offset repeat.

This stitch is easier to learn visually than in written format alone, so if you are new to lattice crochet, consider working through the practice swatch slowly, row by row, referring to a diagram if needed. Many crocheters find it helpful to use stitch markers on the corners of each diamond in progress so they can track where each arm of the lattice is heading.

Pattern Instructions

Foundation and Bodice

The gown begins at the top of the bodice and is worked in the round with a seam at the center back.

Begin by chaining 120 (adjust in multiples of 8 for size). Join with a slip stitch to form a ring, being careful not to twist the chain.

Round 1: Ch 3, dc in each ch around. Join with sl st to top of ch-3. (120 dc)

Rounds 2 through 5: Ch 3, dc in each st around. Join. (120 dc)

These first five rounds form the bodice band. This section is worked in solid double crochet to give the top of the gown a clean, stable edge that will support the weight of the skirt below.

Round 6 begins the transition into the diamond lattice stitch. At this point, you will stop working in solid dc and begin the lattice pattern. Work the first row of the lattice as follows.

Round 6: Ch 5, sk 2 dc, dc in next dc, ch 2, sk 2 dc, dc in next dc. Repeat around, joining with sl st to 3rd ch of beginning ch-5.

Continue working the full diamond lattice repeat as established through rounds 6 to 30, completing approximately 3 full diamond repeats vertically. This section covers the midriff from just below the bust to the natural waist. Work even, keeping your tension consistent. The lattice creates natural negative ease, so the fabric will cling lightly to the body in this section, which is exactly what you want for the mermaid silhouette.

Waist Shaping

At round 31, begin the waist shaping. You will decrease gently here to bring the gown in at the natural waist before it flares back out over the hips.

To decrease within the lattice pattern, work the chain-2 spaces as chain-1 spaces in a strategic sequence every other round for 8 rounds. This subtle tightening pulls the fabric inward without distorting the diamond shapes. Place markers at the side seams (approximately at the 60-stitch mark on each side) to keep your shaping symmetrical.

After the 8 decrease rounds, work 4 rounds even at the tightest circumference. This is your waist point.

Hip Shaping

Beginning at round 44, you will reverse the shaping and begin working increases to accommodate the hips. Mirror the decrease sequence from the waist shaping by working the chain-1 spaces back out to chain-2 spaces, and then adding additional increase rounds as needed to reach the desired hip circumference.

For the standard size medium, the hip circumference at the widest point should measure approximately 40 to 42 inches after blocking. Adjust by working more or fewer increase rounds as your measurements dictate.

Continue through round 60, by which point the gown should sit at approximately the upper thigh. Work 4 rounds even here before beginning the mermaid flare section.

The Mermaid Flare

This is the most exciting section of the pattern and the part that transforms a fitted dress into a genuine mermaid gown. Beginning at round 65, you will work dramatic increases that open the skirt outward in a wide, sweeping flare.

Round 65: Ch 3, 2 dc in each ch-2 sp around, working dc in each dc across. This doubling of stitches begins the rapid flare. Continue increasing in subsequent rounds by working 3 dc in every other ch-2 sp.

Rounds 66 through 75 continue the flare, adding a combination of extra double crochets and transitioning partially back into the lattice stitch to maintain visual continuity. The lattice diamonds in this section are larger and more open than in the bodice, which creates a sense of the pattern blooming outward as it reaches the hem.

Round 76 onward: Work the lattice with a ch-3 between stitches instead of ch-2 to open the diamonds further and add volume to the hem.

Continue until the skirt reaches your desired length. For a floor-length gown with a small train, work through approximately round 90 to 100. Try the gown on regularly during this phase — hem length is deeply personal, and it is much easier to add rows than to take them away.

The Hem Finish

The hem of this gown is finished with a scallop border that echoes the curve of a mermaid’s tail. Work the following border across the final round of the skirt.

Hem Round 1: Join yarn in any dc, ch 1, sc in same st, ch 3, sk 2 sts, sc in next st. Repeat around. Join.

Hem Round 2: Sl st into ch-3 sp, ch 3, 4 dc in same sp, ch 1, 5 dc in next ch-3 sp. Repeat around to create shell clusters.

Hem Round 3: Sc in each dc and ch-1 sp around. Join. Fasten off.

This three-round scallop hem gives the bottom of the gown a delicate, finished look that plays beautifully against the geometric severity of the diamond lattice above it.

Finishing the Gown

After completing the final hem round, weave in all ends carefully. For a garment like this, take the extra time to weave each end in multiple directions so nothing works loose during wear.

Block the finished gown thoroughly. Fill a bathtub with cool water, submerge the gown completely, and gently press out the water without wringing. Roll the gown in a clean dry towel to remove excess moisture, then lay it flat on a blocking surface or hang it carefully to dry. Use blocking pins or wires along the hem scallops to encourage them to open evenly. Allow to dry completely, which may take 24 to 48 hours depending on your yarn fiber and ambient conditions.

Once dry, try the gown on and assess the fit. If you would like to add a ribbon or elastic waist casing for additional structure, do so now by threading ribbon through the waist section of the lattice or sewing a separate casing inside.

Sizing Notes

To adjust this pattern for larger or smaller sizes, modify the foundation chain by adding or removing multiples of 8 stitches. Every 8 stitches added to the foundation chain adds approximately 1.5 inches to the circumference at gauge. Adjust the number of waist shaping and hip shaping rounds proportionally to match your specific measurements.

Always measure your own body at the bust, natural waist, hips, and from the natural waist to wherever you want the hem to fall. These four measurements are the only guide you actually need for fitting a mermaid gown in any size.

Styling Your Midnight Navy Diamond Lattice Mermaid Gown

The midnight navy colorway lends itself beautifully to dramatic accessories. Consider pairing the finished gown with silver or gold statement jewelry, which plays off the deep blue the way stars play off a night sky. For shoes, metallic strappy heels or even bare feet work perfectly depending on the setting. If you are making this gown for a photo shoot or event, a simple updo hairstyle allows the bodice and neckline to take center stage.

For a different mood entirely, this pattern can also be worked in deep emerald, rich burgundy, or a tonal gradient that moves from dark to light as the skirt falls toward the floor. The diamond lattice is a wonderfully versatile canvas.

A Few Final Tips

Always work in good lighting when crocheting with dark yarn. Midnight navy is gorgeous but genuinely difficult to see in dim conditions, and dropped stitches love to hide in dark fabric. A daylight lamp or working near a window will save you an enormous amount of frustration.

If you lose track of your place in the lattice repeat, count the diamond peaks along the last completed row. Each peak is one complete diamond, and counting them helps you re-orient quickly without having to frog back rows.

Save your gauge swatch. If you need to make repairs later, having a piece of matching fabric already blocked and ready is invaluable.

Most importantly, enjoy this project. A mermaid gown is not a quick weekend make, but every hour you invest in it pays off in the finished piece. Work steadily, trust the process, and know that what you are building is something genuinely extraordinary.

Happy crocheting.