DC Young Fly is graciously asking the person responsible for swiping his bookbag to return it pronto ... because its sentimental value outweighs the financial gain in this caper.

Apparently, DC had a gig at the Hollywood Improv last night but discovered his bag had gone missing following the let-out of the show -- and he suspects it got taken by a thief.

The comedian/actor hoped on IG this morning with a passionate plea for the bag to be returned to him ... and it's incredibly sad, because he says there was something inside that was of the utmost importance to him.

According to DC, no cash or jewels were taken but, as it turns out, the backpack actually contained the death certificate of his girlfriend, Jacky Oh!, with whom he had 3 children.

As we reported ...Jacky died last May due to complications from cosmetic surgery and like we told you, no foul play was suspected in her death. It was just a tragic accident.

DC's not asking for anything crazy here ... he literally just wants his bag back, and is hoping his message reaches the right people and that they'll do the right thing and return it.