Provided is a flame-retardant fiber having improved light resistance and/or colorability. The flame-retardant fiber is a fiber including a fiber-forming polymer. The fiber-forming polymer contains a polymer blend comprising a polyetherimide polymer (A) and a thermoplastic polymer (B) having a glass transition temperature lower than that of the polymer (A); and the fiber-forming polymer further contains a functional additive (C) that provides light resistance, colorability, or both of light resistance and colorability. The mass ratio (A)/(B) of the polymer (A) to the polymer (B) is from 50/50 to 90/10, and the content of the functional additive (C) is from 0.5 to 10% by mass relative to 100% by mass of the total mass (A + B + C) of the polymer (A), the thermoplastic polymer (B), and the functional additive (C).