The 27-year-old Buelter has played the season on loan from third tier Magdeburg and will remain for next year, regardless of whether or not Union survive in the top flight.
“The step to Union (Berlin) last summer was exactly the right one,” Buelter told the club website.
“For me personally it’s been an unbelievable first Bundesliga year in which I’ve collected many new experiences and played my part in a good campaign until now.”
Buelter’s seven goals in 28 matches make him Union Berlin’s second top scorer this season.
They are 14th, four points clear of the relegation play-off place, but are winless in six games.
Union Berlin were hammered 4-1 at Borussia Moenchengladbach on Sunday and host Schalke 04 next weekend, where they will be without Norwegian full-back Julian Ryerson.
Ryerson has been ruled out for the season after injuring his knee at Gladbach just 40 seconds after coming off the bench.