You’ll have the most fabulous infant in the neighborhood with this no-sew mermaid baby costume! With inexpensive HeatNBond adhesive and your favorite pair of scissors—your little princess will be ready for a magical night under the sea!

This Halloween costume couldn’t be easier to make—break out your felt stash then prepare some simple shapes—you’ll iron-on your baby’s nighttime debut in no time at all.

You can make this costume as simple or elaborate as you like. It’s just a matter of cutting and pasting and of course, the magic of our go-to adhesive—Therm-O-Web HeatNBond!

We used onesies sized 6–9 months, but the beauty of this craft is that you can adjust the pieces to fit any size!

Baby Mermaid Costume
  • onesies (we used size 6–9 months)
  • 1 – 12″ x 15.5″ piece stiff green felt
  • ink pen or marker
  • assorted shades of green, white and pink felt scraps
  • [url href=”″ target=”_blank” title=”HeatNBond”]HeatNBond[/url]
  • fabric scissors
  • iron
  • hot glue gun and hot glue
  • rhinestone embellishments, if desired
  1. Place stiff green felt on top of onesie then freehand a fish-shaped tail using ink pen or marker. The freehand method will make the shape look more realistic. Cut out tail shape using fabric scissors then set aside.
  2. Cut half-circle ‘gills’ out of assorted shades of the green felt scrap fabric, as needed.
  3. Trim a piece of ribbon the width of the onesie waist then set aside.
  4. Preheat iron to medium heat, no steam. Place adhesive on back of the fish tail to be bonded—paper liner should face up. Press and hold iron on paper liner for a few seconds then allow it to cool.
  5. Trim off excess paper then peel off backing. Place material adhesive side down, to attach fish tail shape on onesie. Press and hold iron approximately 8 seconds on each section, one at a time, until all the pieces have bonded. Repeat with dark green strip of felt at the waist, then flip onesie over and repeat with the green leaves.
  6. Set onesie aside to cool. Meanwhile, trim various circles, about the size of a nickel to the size of a quarter, from the pieces of white and pink scrap felt to make a pearl necklace then set aside.
  7. Next, layer ‘gills’ on fish tail then secure in place using a glue gun and hot glue. Once the ‘gills’ are in place, hot glue the ribbon waistband to the onesie to hide the seam of the fish tail.
  8. Hot glue pearl necklace around the collar of the onesie, then set aside to let the components cool and dry—that’s all there is to it!
Be sure not to use steam when fusing the adhesive to the materials with the iron—it will just make fabric soggy and difficult to stick!