General manager Sam Presti announced the new deal before the team's season opener against the Sacramento Kings on Wednesday night. Financial terms were not disclosed.
"It's definitely a great feeling to be able to say, 'OK, it'll be four more years here' and everything," Sefolosha said. "For the team to offer me a contract like that is great."
Sefolosha and his agent, Guy Zucker, said the contract is guaranteed for four years with no options. The Swiss guard was entering the final year of his initial rookie contract, signed after he was drafted by Philadelphia with the 13th overall pick in 2006.
Sefolosha said he didn't consider the prospect of being a restricted free agent daunting but was glad to find stability with the Thunder, who traded a first-round pick to Chicago in February to pick up the guard who had fallen into a diminished role with the Bulls.
"I was looking forward to it, but I love the environment here and just the opportunity to stay with the team. I really like that," Sefolosha said. "We've got a great team, teammates I really enjoy. Just being here for a few more years, I like it."
Sefolosha averaged 8.5 points and 5.2 rebounds last season with the Thunder after the trade, starting 22 games in place of the injured Desmond Mason.
"We're looking forward to having him with us," coach Scott Brooks said. "He's about everything that we are about. He wants to get our team better, he wants to improve himself and he's a defensive-minded guy. He's going to help us. He helps us in practice and he helps us in games."
Sefolosha, 25, is married and has three young children. He said the stability for the family is "something that also pushed me to do this this year."
"It's a relief," Sefolosha said. "It definitely feels good just calling the family and telling them, `OK, it's done and I'm going to be in the NBA for a few more years."
Zucker said the deal came together quickly in a 24-hour period after team chairman Clay Bennett "stepped up and improved the offer significantly." He credited Presti for "pushing it and pushing it" to get the deal done.
"It's a fully guaranteed deal," Zucker said. "He was seeking stability and longevity and so were they."
Presti preferred not to discuss how the two sides arrived at the extension terms. Sefolosha said he signed the contract Wednesday after the team's morning shootaround.
"I don't comment on negotiations for that reason, because there's always ebbs and flows any time you're working on an agreement," Presti said. "I think what you have here is, an important part to note is that Thabo really wanted to be here. Thabo wanted to be a part of our organization and to continue working together with group that we have."
Sefolosha plays the same position as James Harden, the Thunder's No. 3 overall draft pick this year, but Presti said the team likes having both players because each can play multiple positions.
"He's another core piece that we feel like now can continue to grow with the group that we have," Presti said. "I think he can improve in all areas of his game, and one of the reasons I'm confident of that is because he's a high character guy and a worker. Put that in place with someone that's gifted like he is athletically and defensively and an improving offensive player, I would expect him to improve his basketball."