is an English professional former player and football manager who is the manager of EFL League Two club Gillingham.

A former youth player at Blackburn Rovers, the midfielder, who was known for his crossing ability, moved to Preston North End in 1992 after impressing at non-League Northwich Victoria. He signed for Cambridge United, who in turn loaned him back to Northwich, and returned to Preston in 1993, establishing himself in the first-team during his second spell. Sold on to Lincoln City in 1995, his performances earned him a place on the PFA Third Division Team of the Year in 1996–97, resulting in a £500,000 move to Port Vale. After being named the Port Vale F.C. Player of the Year in 1998, Ainsworth was sold to Premier League club Wimbledon for £2 million. Injuries at the club dogged him, and following loan spells with Preston and Walsall, he moved to Cardiff City in March 2003. He signed with Queens Park Rangers in June 2003, where he spent seven years. During his time at QPR, he helped the club to promotion from the Second Division in 2003–04, and twice served as caretaker manager.

Following a short loan period, he joined Wycombe Wanderers in February 2010. He was named in the PFA League Two Team of the Year in 2010–11, as Wycombe won promotion. After a short period as caretaker manager, he was appointed manager in November 2012. He retired from regular playing appearances to concentrate on management in April 2013 but remained registered as a player. He led the club to promotion from League Two at the end of the 2017–18 season and then from League One in 2020, securing a place in the Championship for the first time in the club's history. He left Wycombe in February 2023 to become head coach of QPR, where he would remain for eight months. He was appointed head coach at Shrewsbury Town in November 2024 and switched clubs to Gillingham after five months.