The veteran actor and "Milkshake" singer recently started hitting it off -- according to the US Sun -- which reports they've been hanging out a lot together, both in the United States and across the pond.

While it may sound like an unlikely pairing at first glance, 72-year-old Bill recently took in a couple of 43-year-old Kelis' shows in London ... posing for a backstage photo together after at least one of the gigs.

Not only that, they were reportedly also seen at the same London hotel, and have been getting close since linking up stateside.

There is a common bond between them, though it's not exactly cheery ... Kelis' second husband died in March 2022 and Bill's estranged wife died in 2021, and that's one of the things they reportedly bonded over.

It doesn't sound like Bill and Kelis are putting labels on their situation ... the outlet cited a friend as saying, "Whatever it is that has brought them together, and however unlikely it seems, they are both single and are having fun despite the fairly big age gap."