“As far as I'm concerned I'm very thankful that he let go of a large number of people because now we have a smaller payroll,” Trump said at a briefing at his golf club in Bedminster, New Jersey on Thursday.
"We'll save a lot of money," he added.
It's unclear if the U.S. would save any money because of the expulsion. Any diplomats removed from Russia would be sent to posts elsewhere around the world.
A State Dept. official who is a foreign service officer told ABC News the message from Trump thanking Putin is "really quite sad."
"I'm not even angry, it's just saddening," the official said, who spoke on the condition of anonymity. The official cited what they say is the perception among State Dept. employees that Trump does not support the institution or its diplomats abroad and a sense that "he just doesn't get it."
A former U.S. ambassador also noted a pattern in the comments: "For reasons we do not yet know, the President cannot bring himself to criticize Putin."
The comments were swiftly condemned on a bipartisan basis, too -- by foreign policy voices across the political spectrum.
The longtime Middle East diplomat and negotiator Aaron David Miller tweeted, "Having served at State for 25 yrs under R/Ds, Trump's defense of Putin over expelled US diplomats one of most shameful of his presidency."
"If in a generous mood, you could argue that POTUS is showing Putin that he isn't bothered by this," he told ABC News.
Tillerson has said the U.S. will decide how to respond to the expulsions by Sept. 1.
ABC News' Chris Donovan contributed to this report.