MPC1 and MPC2 are two well-known components of the mitochondrial pyruvate carrier (MPC) complex maintaining MPC activity to transport pyruvate into mitochondria for tricarboxylic acid (TCA) cycle entry in mammalian cells. It is currently unknown whether there is an additional MPC component crucially maintaining MPC complex activity for pyruvate mitochondrial import. Here we show that ALDH4A1, a proline-metabolizing enzyme localized in mitochondria, serves as a previously unrecognized MPC component maintaining pyruvate mitochondrial import and the TCA cycle independently of its enzymatic activity. Loss of ALDH4A1 in mammalian cells impairs pyruvate entry to mitochondria, resulting in defective TCA cycle entry. ALDH4A1 forms an active trimeric complex with MPC1-MPC2 to maintain the integrity and oligomerization of MPC1-MPC2 and facilitates pyruvate transport in an in vitro system. ALDH4A1 displays tumour suppression by maintaining MPC complex activity. Our study identifies ALDH4A1 as an essential component of MPC for pyruvate mitochondrial import, TCA cycle entry and tumour suppression.