For some reason, the lawyer repping the actor has filed the lawsuit under the fictitious name, John Doe. The actor is suing Lisa Ragland -- claiming she broke off their engagement in 2003 but REFUSED to return the $90,000 engagement ring.

Sources connected to the case tell us "John Doe" is not Ian Ziering, who was also engaged to Ragland around the same time. Our sources tell us ... it's a "huge actor."

According to the lawsuit, after Ragland called off the engagement, she told the actor she lost the ring ... and he believed her.

That is, until 2010 ... when she fessed up and admitted she had the ring all along. So now Doe is suing her for either return of the ring or its full cash value.

Here's the deal -- under California law, if an engagement is called off, the guy always gets the ring back ... and it doesn't matter who broke it off. The ring is considered a "gift in contemplation of marriage" ... so if the marriage doesn't take place, the ring goes back.