YouTube star Caleb Marshall, who goes by The Fitness Marshall on social media, is pulling the plug on his marriage ... because he's getting a divorce.

Caleb and his husband, Cameron Moody, beelined it to court last week and filed a joint petition asking the court for a divorce after 3 years of marriage ... according to new legal docs obtained by TMZ.

The reason for the split is unclear.

Caleb and Cameron list the date of marriage as February 2, 2023 ... but they do not list a date of separation.

The joint filing says they agree on spousal support, division of property, legal fees, and the date of separation. They did not have any children during the marriage, so no issues with child support.

Caleb's got a huge following online, where he livestreams his workouts.