Al Jardine may need a trip to Kokomo ... 'cause the Beach Boys star is dealing with a legal headache after being sued for allegedly refusing to sign merch.

Here's the deal ... a man named Mead Chasky is suing the Beach Boys crooner, alleging Jardine agreed to sign merchandise advertised as "signed by three members of The Beach Boys."

However, according to Mead's attorney, Matthew J. Blit, Jardine refused to sign the memorabilia ... violating his contractual obligation.

Chasky is suing Jardine for $100K ... plus, he wants the singer to cover his legal fees.

Jardine co-founded the Beach Boys in 1961 alongside Brian, Dennis and Carl Wilson and their cousin Mike Love. He's still a member of the group ... playing with them periodically for more than 60 years.

Of course, there has been some infighting between members of the band in recent years ... though Jardine released a single, "Wish" last year and dedicated it to longtime bandmate Brian Wilson. He also donated part of the proceeds to the Make-A-Wish Foundation.

We've reached out to Jardine. So far, no word back.