Sigmar Gabriel, who served in the top post until earlier this year, told German media that the US president “only knows strength” and accused him of wanting “regime change” in Berlin.
Mr Trump is currently on a seven-day tour of Europe, leaving political discord in his wake. On Friday he attacked Theresa May’s Brexit strategy, questioned whether he would do a trade deal with the UK, and backed the PM’s rival Boris Johnson as future prime minister – despite her efforts to form an alliance.
“Every step to accommodate him, every attempt to please, only leads to him to take a further step. This needs to end,” Mr Gabriel, a former coalition partner of Ms Merkel, told Der Spiegel magazine.
“America under Trump cannot be relied upon. Trump is giving the North Korean dictator a survival guarantee and wants regime change in Germany at the same time.”
There was outrage in Germany last month after Mr Trump’s newly appointed ambassador to Ms Merkel’s government said he saw it as his job to “strengthen” right-wing movements across the continent.
MPs called for Richard Grenell to be expelled from the country for political activity, a major breach of diplomatic protocol. The German foreign ministry said it had asked the US to clarify its position.
Mr Trump’s intervention against his aspirant ally Ms May on Friday is the latest in a string of attacks on European politicians. The US president has repeatedly accused Europe of trading with the US on unfair terms and has said EU member states need to spend more on defence.
Mr Gabriel called for a “clear, firm, and above all, unified European responses to Trump”, adding: ”We can’t stand for this. We can no longer have any illusions. Donald Trump only knows strength. So we have to show him that we are strong too.”
His comments echo concerns of senior EU officials in Brussels who believe Mr Trump works under “a new American doctrine in which there are no friends and no enemies”.
Donald Tusk, the president of the European Council, warned EU leaders to prepare for “worst-case scenarios” with the US president. This week, ahead of a Brussels Nato summit, he warned the US president to “appreciate your allies: after all you don’t have that many”.
Ms May has made significant and repeated diplomatic overtures to gain the friendship of the US president, inviting him for a state visit despite his unpopularity in Britain, and twice being pictured holding his hand in public.
She has also turned a blind eye to Mr Trump’s admission that he uses his fame to grope women, his stated policy of wanting to ban Muslims from entering the US, and other controversial policies such as separating migrant children from their mothers in detention.