Merkel, 64, has no history of serious health issues. She was attending a farewell ceremony for Justice Minister Katarina Barley, who is leaving to become a lawmaker in the European Parliament.
Berlin is in the throes of a heatwave. As she shook, Merkel crossed her arms as if to brace herself. She was offered a glass of water but declined to drink it while Steinmeier spoke.
On June 18, Merkel was also seen shaking when she met visiting Ukrainian President Volodymyr Zelenskiy but later said she felt better after drinking some water.
Asked if the chancellor would take part in this weekend's G20 meeting in Japan, the spokesman said: "Everything is taking place as planned. The chancellor is well."
After the ceremony with Steinmeier, Merkel went on to the Bundestag lower house of parliament for the swearing-in of the new justice minister. She showed no signs of shaking and looked relaxed, chatting and laughing with Vice Chancellor Olaf Scholz.
Merkel is renowned for her work ethic and has a reputation for outlasting other leaders at European Union summits with her ability to focus on the details of complex discussions deep into the night.
In the past, she has joked that she is a "sleep camel" who can go days with just a few hours of sleep as long as she gets a full night of shut-eye at the weekend.
Merkel has loomed large on the European stage since 2005, helping guide the EU through the euro zone crisis and opening Germany's doors to migrants fleeing war in the Middle East in 2015 — a move that still divides the bloc and Germany.
The chancellor faces a grueling schedule in the coming days.
Later on Thursday, she flies to Japan for the G20 meeting before heading to Brussels for an EU summit on Sunday at which she will play a key role in trying to seal a deal on the distribution of the bloc's top jobs for the next five years.
Merkel appeared her usual self when she fielded questions from lawmakers during an hour-long session in parliament on Wednesday, shortly after which she gave a speech at the Humboldt University of Berlin.
Merkel began a stage-managed gradual exit from politics in October, when she said her fourth term as chancellor would be her last and that she would not seek re-election in 2021, when the next federal election is due.
In December, Merkel handed over the chair of her Christian Democratic Union (CDU) to her protege Annegret Kramp-Karrenbauer, though her unconvincing start in the role has set back the party's plans for a smooth leadership transition.
When Merkel came into office in 2005, George W. Bush was U.S. president, Jacques Chirac was in the Elysee Palace in Paris and Tony Blair was British prime minister.