Actually, it is! It’s even in the online Merriam-Webster Dictionary. Unfortunately, “unfrequently” was surpassed in popularity by its relative “infrequently.”

“Unfreqently” is so unpopular that spell check on my computer keeps switching it to “infrequently,” and Grammarly is displaying a message that says: “This word is unknown.”

I think we should use “unfrequently” more frequently, because “unfrequently” is the more infrequently used word. That means it’s more appropriate in contexts where we want to get the magnitude of infrequentness across to the reader.

In other words, “unfrequently” is a more powerful way of saying “infrequently.”