The deal includes a $10-million option for 2022 and escalators that would increase that year's salary and buyout cost, reports Alex Speier of the Boston Globe.
Richards posted a 4.03 ERA and 1.25 WHIP over 14 games (10 starts) with the San Diego Padres in 2020. He returned from Tommy John surgery in late 2019 after undergoing the procedure in 2018.
The 32-year-old owns a career 3.62 ERA with 47 wins across 10 seasons with the Padres and Los Angeles Angels.
Boston brought back left-hander Martin Perez last week to assist a Red Sox rotation that accrued the lowest fWAR in the majors during the last campaign. The team also reportedly agreed to a two-year contract with utility man Enrique Hernandez on Friday.