'Sugar' Shane Mosley to Ex-Wife -- Read It and Weep ... Docs Show You Two-Timed Me

"Sugar" Shane Mosley firmly believes his ex-wife was married to another man at the same time as him ... because he has papers nailing down the date she divorced the guy.

Sources close to Mosley tell TMZ Sports ... Shane's lawyers obtained a NY State Certificate of Dissolution of Marriage for Jin Mosley, dated Sept. 26, 2006. Shane and Jin were married from 2002 to 2011.

Do the math ... it all adds up to bigamy, as far as Shane's concerned.

We're told Shane presented the divorce doc in court last week ... in hopes of getting a judge to freeze Jin's assests. TMZ broke the story ... Shane's hoping to get back all or some of the cash and assets he forked over to her when they divorced -- since they were never really married, he believes.

We're told the judge has not ruled yet on Shane's request.