You look beautiful/handsome tonight.
'Bellissimo' = bel-LIS-si-mo. The double 's' lengthens the sound slightly. Four syllables.
Use this for any social occasion when someone looks particularly well-dressed or radiant. In Italy, commenting on physical appearance is completely normal and expected — it is a sign of noticing and appreciating the other person.
'Bellissimo/a' is the absolute superlative of 'bello/bello' — formed by adding '-issimo/a' to the stem. It means extremely beautiful. 'Stasera' (tonight) specifies it is about their current appearance, not making a general claim.
Sei raggiante stasera!
You are radiant tonight!
'Raggiante' (radiant) suggests inner joy shining outward — more poetic
Stai benissimo con quell'abito.
You look wonderful in that outfit.
Connects the compliment to a specific piece of clothing
Non ti avevo mai visto/a così elegante.
I have never seen you so elegant.
Implies positive surprise at this particular appearance
In Italy, complimenting someone's appearance is a fundamental social act — connected to the concept of 'fare bella figura' (making a good impression). Italians notice and appreciate when others look well, and expressing this appreciation openly is expected. Not complimenting someone who clearly made an effort can be perceived as indifference.