The photo was snapped on June 16th in a city called Reykjavik -- where Tom's been filming his new movie -- and by the looks of it, Tom wasn't suspecting a thing as far as divorce was concerned ... walking around hand-in-hand with Katie.
And Katie didn't let on either.
The pic is even more proof Tom was "blindsided" by the divorce papers.
TMZ broke the story ... Katie filed divorce docs against Tom in New York yesterday, citing "irreconcilable differences."
The primary difference ... Scientology.