Joe Mixon was so confident he was gonna sign a big NFL contract before the 2017 Draft -- he dropped nearly $200,000 on some insane jewelry ... this according to a lawsuit obtained by TMZ Sports.

Remember, before the Bengals selected the RB with the 48th pick -- there were serious questions on whether a team would pull the trigger based on his violent 2014 incident with a female Oklahoma student.

But Mixon wasn't questioning himself ... and allegedly made at least 3 major purchases in February, March and early April which included the following items: