The 2009–10 season was the 115th season in the history of Plymouth Argyle Football Club, their 85th in the Football League, and 41st in the second tier of the English football league system. Their 21st-place finish in the 2008–09 season meant it was their sixth successive season playing in the Championship. Their campaign ended disappointingly as they were relegated to League One with 11 wins, 8 draws, and 27 losses from 46 matches. The club exited both domestic cup competitions at the first stage, being eliminated by Newcastle United in the third round of the FA Cup and by Gillingham in the first round of the League Cup. The club changed managers in December due to a string of poor results as Paul Sturrock was replaced by Paul Mariner.

The club began their league campaign with two draws, but defeat by Cardiff City marked the beginning of a seven-game losing streak. Results improved in October and November before three more successive defeats saw Sturrock replaced by Mariner. Performances gradually improved and the club won both of their fixtures during the Christmas period, including a 4–1 win against Reading; their best of the season. The club signed Damien Johnson in the winter transfer window from Birmingham City and made a number of signings on loan, including Kenny Cooper and , in order to try to improve their league position. A succession of draws and narrow defeats followed in February, which prompted Mariner to make more signings.

Argyle lost three of their next nine matches to give themselves a chance of avoiding relegation, but their win at Doncaster Rovers proved to be the last game that they would collect any points in. Defeats at home to Middlesbrough and away to Watford left the club on the brink of a return to England's third tier, and that was confirmed in their next game. Newcastle United won 2–0 at Home Park to secure the Championship title, and relegated the home side at the same time. The club finished 23rd in the league table after defeat by bottom club Peterborough United on the final day of the season, eight points adrift of safety. Jamie Mackie finished as the club's top scorer with eight goals in all competitions, and captain Carl Fletcher was voted Player of the Year.

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