Psych!

This is an exclamation with the sense of ‘Gotcha!’ or ‘Just kidding!’ which we have belatedly picked up from American English where it first appeared in the 1980s.  The precursor to it was the verb to psych someone out meaning ‘to fool someone’ in particular by messing with their minds or playing mind games.  Use it where you would have used Gotcha! to be completely with it and up-to-date.