Sparks general manager Penny Toler thanked Agler for his services in a statement on the WNBA team's website: "We'd like to thank Brian for his contributions to the Sparks organization over the last four seasons, including helping the team win the 2016 title. We wish Brian nothing but the best moving forward."
No reason for his resignation was provided. Per the release, the team will immediately begin a "comprehensive search" for its next head coach.
Agler took over in Los Angeles in January 2015 and led the organization to a sustained run of success over the past four seasons. The Sparks made the postseason in all four years under Agler, including back-to-back WNBA Finals in 2016 and '17.
A victory over the Minnesota Lynx in the 2016 Finals gave the Sparks their third title in franchise history. It was Agler's second championship as a head coach after also winning in 2010 with the Seattle Storm.
Two consecutive Finals appearances earned Agler a multiyear extension in September 2017.
The Sparks went 19-15 during the 2018 regular season before being eliminated by the Washington Mystics in the semifinals.
Agler, 60, has spent 15 seasons on a WNBA sideline with those three clubs. He has compiled a 269-220 regular-season record, going 24-19 in 10 postseason appearances.