The visitors produced an impressive first-half performance before surviving a late Union rally to claim all three points. Former Union player Robin Gosens opened the scoring against his old club, while Adil Aouchiche doubled Schalke's advantage shortly after the interval.
• Schalke: Robin Gosens 25', Adil Aouchiche 46', Maximilian Wöber 90+13'
• Union Berlin: Tim Skarke 90+6'
Schalke started brightly and created the better opportunities during the opening stages, with Kenan Karaman going close before Aouchiche was denied by Union goalkeeper Frederik Rønnow.
The breakthrough arrived in the 25th minute when Gosens, returning to the stadium where he previously played for Union, struck against his former side. His effort went in off the inside of the post to give Schalke a deserved lead.
Union struggled to find a response before the interval, although Rønnow kept the hosts within touching distance with important saves.
Schalke then struck again almost immediately after the restart. Aouchiche fired a powerful effort under the crossbar in the 46th minute following good work from Junior Adamu, giving the visitors a commanding 2–0 lead.
Union responded with a sustained spell of pressure. The hosts hit the woodwork twice and repeatedly tested Loris Karius, who produced another impressive display after his outstanding performance in Schalke's goalless draw with Bayern Munich the previous weekend.
The pressure finally paid off in the sixth minute of stoppage time. Tim Skarke collected Tom Rothe's pass and pulled one goal back for Union, setting up a tense finish.
But Schalke had the final word.
With the match deep into the 13th minute of added time, substitute Maximilian Wöber completed a swift counter-attack, finishing from close range after Hee-chan Hwang's low delivery to seal a 3–1 victory.
The victory moved Schalke to four points from their opening three Bundesliga matches, having previously lost to Augsburg and held Bayern Munich to a 0–0 draw.
For Union Berlin, the defeat extended their difficult start under Mauro Lustrinelli. The Berlin club remain on one point after three matches and have now conceded 10 goals in the opening three league games.
The match was also briefly interrupted after referee Timo Gerach suffered a calf injury and was unable to continue. Fourth official Jarno Wienefeld took over, making his Bundesliga refereeing debut.
Schalke will now look to build on their first league win when they face fellow promoted side Elversberg, while Union face another difficult assignment against Bayern Munich next Friday.