Brought in over the summer to replace Nottingham Forest-bound Taiwo Awoniyi, Siebatcheu glanced a Sheraldo Becker cross past Oliver Christensen to put Union ahead in the 32nd minute.
Becker added the second goal in the 50th minute, before Robin Knoche headed in a Christopher Trimmel corner four minutes later to put the result beyond doubt.
Hertha winger Dodi Lukebakio scored a late consolation goal.
Union have now won four in a row against their big brothers from the city’s west and have only lost twice from seven Bundesliga clashes since being promoted to the top division in 2019.
The loss comes a week after Hertha, who narrowly escaped relegation in 2021-22, were eliminated in the first round of the German Cup by second division Eintracht Braunschweig.
Speaking before the match, Union Berlin coach Urs Fischer lauded his new arrival, saying Siebatcheu, who will look to lead the line for the United States at the Qatar World Cup, “might not have the speed (of Awoyni), but he certainly feels at home in the penalty area.”
After the game, Fischer took a pragmatic approach, saying the most important aspect of the derby victory was securing the three points, rather than bragging rights in the German capital.
“For the fans it is an important game, but me, I am a bit more pragmatic and realistic,” the Swiss coach said.
“For me, it’s about the three points. When you win games, you gain trust in yourself and (improve your) self-confidence.”
Hertha coach Sandro Schwarz said the derby loss was “irritating” but that his side had the quality to bounce back next week.
“We have the possibility to show a different face next week. Right now it hurts, there’s no doubt.”
Werder Bremen celebrated their return to the Bundesliga after a year in the second division by snatching a point in a 2-2 draw away at Wolfsburg.
– Kruse control –
In their first Bundesliga match under former Bayern Munich and Monaco coach Niko Kovac, Wolfsburg took the lead early, with Lukas Nmecha dinking a perfect chip over keeper Jiri Pavlenka to put the home side in the lead.
Bremen, clad in a salmon pink away strip, hit back with two quick goals through Niclas Fuellkrug (21st minute) and Leonardo Bittencourt (23rd) to take a lead into halftime.
Kovac brought on Max Kruse in the 76th minute with his final change.
Kruse, who scored 35 goals in 94 games with Werder, quickly created an equaliser, gliding a cross into the penalty box for Josuha Guilavogui to hammer it home.
Elsewhere, Borussia Moenchengladbach got off to a winning start under new manager Daniel Farke, beating 10-man Hoffenheim 3-1 at home.
Hoffenheim took the lead in the 25th minute through Robert Skov just after defender Stefan Posch received his second yellow for a studs-up tackle on Ramy Bensebaini, but were unable to hold on.
The visitors conceded goals to Bensebaini (41st minute), birthday boy Marcus Thuram (71st minute) and Nico Elvedi (78th minute).
Beaten German Cup finalists in 2021-22, SC Freiburg continued their strong form under long-time manager Christian Streich, scoring four second-half goals to win 4-0 away at Augsburg.
Michael Gregoritsch scored just seconds after halftime, before dead-ball specialist Vincenzo Grifo added another to put Freiburg 2-0 up in the 47th minute.
Summer arrival Matthias Ginter added a third in the 61th minute, before fellow new recruit and Japanese international Ritsu Doan added a fourth in the 78th.
Mainz defeated Bochum 2-1 through two goals from Austrian striker Karim Onisiwo.
On Friday night, defending champions Bayern Munich got their quest for an 11th successive title off to the best possible start, routing Europa League champions Eintracht Frankfurt 6-1.
Borussia Dortmund will look to keep pace with Bayern when they take on Bayern Leverkusen in Saturday’s late game.