The short answer: a low-back bra with a converter clip for a moderate low back, or stick-on silicone cups for a fully open back, depending on how low the dress actually dips.
For a Moderate Low Back
Most bras with convertible straps can clip low using a low-back converter, a small hook that pulls the band down. This works when the back dips to the mid-back but doesn't go fully open.
For a Fully Open Back
Once the back drops past where a bra band would normally sit, stick-on silicone cups or fashion tape are the only options that stay invisible. Reusable silicone cups hold up better through a full night than one-time adhesive strips.
For Smaller Busts
Fashion tape alone, applied in an X or parallel strips, is often enough support without any cup at all.
What to Avoid
Don't try to modify a regular bra by unclipping the back band, it won't hold and defeats the support entirely. Buy for the specific dress, not a workaround for the bra you already own.
Pairing It With Your Dress
Low-back and open-back styles are cut to work with low-back bras or stick-on options without bunching or peeking through the sides.
