Trump said the likes of France had been more supportive and he had never expected to see the once "most solid of all" relationships change in this way.
"It's very sad to see that the relationship is obviously not what it was," Trump told the Sun newspaper on Tuesday, his second interview to a British newspaper in as many days where he criticised the British leader.
Starmer said late on Sunday he would allow the US to use British military bases for defensive strikes after they were not used in the initial attack on Iran.
Trump said the US did not need Britain to wage war in the Middle East but added: "It's not going to matter, but (Starmer) should have helped… he should have.
"I mean, France has been great. They've all been great. The UK has been much different from others."
Senior British minister Darren Jones told Times Radio in response to the latest Trump comment's that the US-UK relationship remained crucial but said the country had learnt lessons from its involvement in the 2003 Iraq war.
"One of the lessons of Iraq was that it's better to be involved in these situations when you are aligned with international partners, and as I say, with a clear legal basis in the plan," he said.
Starmer said in parliament on Monday: "President Trump has expressed his disagreement with our decision not to get involved in the initial strikes, but it is my duty to judge what is in Britain's national interest. That is what I have done, and I stand by it."
Iran has hit targets across the Middle East, including in the United Arab Emirates, Qatar and Saudi Arabia, and there are fears Tehran has the ability to keep up its retaliatory actions for a week.
Around 300,000 Britons are believed to be in countries targeted by Iran, with 102,000 registering their presence with the Foreign Office as officials worked on contingency plans including a possible mass evacuation.
Starmer said the government was "looking at all options to support our people".
Hours after Starmer's announcement on Sunday about allowing the US use of bases, a drone hit RAF Akrotiri in Cyprus.
- with PA
