β-Amino acids serve as crucial building blocks for a broad range of biologically active molecules and peptides with potential as peptidomimetics. While numerous methods have been developed for the synthesis of β-amino acids, most of them require multistep preparation of specific reagents and substrates, which limits their synthetic practicality. In this regard, a homologative transformation of abundant and readily available α-amino acids would be an attractive approach for β-amino acid synthesis. Herein, we disclose the development of a sequential process to provide diverse β-amino acids from α-amino acid derivatives and commercially available phosphonium ylides via visible light photoredox catalysis. In this two-step protocol, phosphonium ylides function as a bifunctional linchpin: they act as a carbon nucleophile to forge a C-C bond in the first step and as a carbon-centered radical source for diverse modifications of the β-amino acid scaffold in the second step. The orthogonal activation of these reactivities under mild photocatalytic conditions enables a modular three-component assembly to access β-amino acids and dipeptides with high structural diversity.