He beat defending champion Aidan Zittersteijn (North Taranaki) 10-9 in a close singles final.
The scores were level at 8-8 after 10 of the 12 ends.
Horwell dropped a shot on the penultimate end but came back strongly to gain two shots on the final end to win the game.
``I planned ahead to make sure I covered all Aidan's shots,'' Horwell told the Otago Daily Times from Wellington.
``On a few ends Aiden got the shot with his last bowl to keep in the game.''
Horwell (17), an apprentice electrician, also won the junior singles in 2014. He won the New Zealand secondary schools singles title last year.
Horwell beat Daniel Curtis (Manawatu) 12-6 in one semifinal and Zittersteijn eliminated Leighton Shanks (Poverty Bay) 13-8 in the other.
``It is good to be able to back up the titles,'' Horwell said. ``It felt good winning my first one but winning another one showed it was no fluke.''
Horwell has also won national titles in outdoor bowls and was the New Zealand secondary schools singles champion in 2014.
He also played in the singles for the winning Otago sevens team in a national competition for bowlers with less than eight years' experience.
He plays for Otago in the national final of the Paterson Trophy for the first time in Christchurch next weekend.
Daniel Sinclair won the singles title at the young guns aged from 18 to 30 indoor bowls tournament in Hamilton at the weekend.
Sinclair (26), who has played indoor bowls for 14 years and has seven Otago titles, beat Alex Reed (Auckland) 11-9 in the final.
