noun
fortunate person; lucky fellow
Refers to someone who is happy or lucky, often with a slightly envious or affectionate tone. Can be used sincerely or playfully.
彼は本当に幸せ者だね。
He's a really lucky guy, isn't he?
こんなに友達に恵まれて、私は幸せ者です。
I'm a fortunate person to be blessed with such friends.
幸せ is the abstract noun/adjective meaning 'happiness' or 'happy', while 幸せ者 specifically refers to a person who is happy or lucky.
Compound of 幸せ (happiness, good fortune) and 者 (person). The reading しあわせ is a sound shift from しあわせ (仕合わせ), originally meaning 'coming together' or 'coincidence', which developed a positive connotation.