Only Ted C. MacRae ventured a guess for this most recent identification challenge. He was exactly right, though a bit confused by his source which indicated this species might not occur in Costa Rica. This is indeed Phiale guttata.
The World Spider Catalog lists the distribution for this wide ranging species as Costa Rica to Paraguay. The Global Species Database of Salticidae site lists all the species for Costa Rica, and allowed me to eliminate the possibility of similar spiders in the same genus. There are also some good photos and illustrations there.
This specimen was impressively large, perhaps the largest salticid I’ve ever encountered. A snippet from “A mimicry complex between mutillid wasps (Hymenoptera: Mutillidae) and spiders (Araneae)” states that this is the largest Salticid from the study area. I can’t access the article, but the title suggests this species might mimic velvet ants (Mutillidae). The snippet also mentions two color morphs, yellow and black as here, as well as red and black. It must be quite variable, because there’s a photo here of a white and black morph as well.
This female’s abdomen appears even larger than normal. I’d guess she’s gravid.
She was quite photogenic, so here’s some more photographs.