Usain Bolt and his GF are still going strong ... the two hit up Cabo this week -- and the bikini she was rockin' and the mojitos they were drinkin' say they're doing just fine.
Of course ... Bolt and Kasi Bennett have had a very unique relationship over the years -- 'memba the EPIC hall pass she allegedly gave him during the 2016 Rio Olympics??
But, doesn't seem all that other tail has put a strain on their relationship ... 'cause the two soaked in the Mexico rays just fine on Thursday.