Gender neutral term for "maiden name"?

Birth name is completely gender neutral, but it might be confusing out of context.

Pre-married name will provide context in addition to being gender neutral.

Or even née and né which are derived from French meaning born. They are past participle of naître (to be born). But, as most Americans wouldn't recognize né as masculine and née as feminine, it retains some of its neutrality. You would still need to designate male and female properly for those who do, but it won't be as loaded as "maiden name".

Whilst I agree with the previously answered "birth name", I'd also like to offer the simple "previous name".

A lot of forms I have completed will have forename, surname, and previous name fields.

It's gender-neutral and it covers most scenarios - same-sex marriage, change through adoption, change through divorce, change through deed poll, etc.

Another option: Surname at birth. Applies to both women and men, as couples can take a hyphenated married name.

The term "maiden name" clearly indicates when the name was changed (when the bearer ceased to be a maid) and why (because she became a wife).

If you say "previous name" or "birth name" we are left wondering when he changed his name and why. The suggestion "pre-married name" sounds too much like "pre-washed" or "pre-adjusted" and that makes it confusing.

They are just two short to explain what will be an unfamiliar concept to much of your audience. Do not try to invent a term. Instead select a few simple words. You could say, "He name before he was married was Smith."