Boric acid is an acid because its molecule:

Boric acid (H3BO3) is a weak monobasic acid that behaves uniquely compared to traditional Brønsted-Lowry acids. Instead of donating a proton (H+) through direct dissociation, it acts as a Lewis acid by accepting an electron pair from a hydroxide ion (OH-) in water [1]. When boric acid is added to an aqueous solution, it interacts with water molecules to form the tetrahydroxyborate ion [B(OH)4]- [1]. This process involves the boric acid molecule accepting a hydroxide ion from water, which consequently releases a proton (H+) into the solution. This release of a proton from the water molecule, triggered by the acceptance of OH-, is what accounts for its acidity [1]. Therefore, it is classified as a Lewis acid because its boron atom is electron-deficient and can accommodate an additional pair of electrons from the hydroxide ion.

Sources

1. [1] https://www.sciencedirect.com/science/article/abs/pii/S0022354920302082