The 75-year-old man died on Wednesday, a day after the accident on Tuesday. Police have appealed for witnesses.
The collision occurred on the B4568 Aberhafesp to Llanwnog road at the crossroads with the B4569 near to Caersws at around 2.45pm," said a police spokesperson.
A Vauxhall Astra and a Volkswagen Passat were involved.
“Sadly, a 75-year-old man died the following day. Anyone who saw the two cars driving along this road at around 2.45pm, or who witnessed the collision, is asked to contact Pc1169 Mark White at Newtown Police station.”