Horstmann took the top award at Warriors for the second time after also winning the gong at the end of the 2006/07 season.
The 27-year-old was also named the Player's Player of the Year by his fellow team-mates with Matt Mullan named runner-up and centre Dale Rasmussen also shortlisted.
"It's a fantastic honour to pick up the two awards handed out by the coaching staff and my fellow team-mates at Warriors," said Horstmann.
"I have really enjoyed this season and I'm already looking forward to coming back next year and looking to get even better to help push Warriors up the Guinness Premiership table."
Prop Mullan, who was this year was selected in the England Saxons squad, was named the Most Improved Player ahead of runner-up scrum half Ryan Powell and wing Charlie Fellows.
Centre sensation Alex Grove was named Young Player of the Year after his big breakthrough season at Sixways Stadium. Lock Graham Kitchener was named runner-up and Jonny Arr was also praised.
The inaugural Warrior of the Year award was handed to talismanic skipper Pat Sanderson with Greg Rawlinson runner-up and Dale Rasmussen third.