“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.
The Russian order to expel 755 U.S. diplomats last month came in response to new sanctions imposed by the U.S. A diplomatic row over Russian interference in the 2016 election began last year after President Obama ordered the seizure of two diplomatic facilities used by Russia in the U.S. and the expulsion of 35 diplomats and their families.
One of those compounds, in rural Maryland outside of Washington, was said to be used for espionage, according to former Director of National Intelligence James Clapper.
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.
Harvard University professor Nicholas Burns, who was ambassador to NATO and Under Secretary of State for Political Affairs under President George W. Bush, called Trump's comments disrespectful.
As a Foreign Service veteran, I find it lamentable that our great career diplomats are treated with such disrespect by their President— Nicholas Burns (@RNicholasBurns) August 10, 2017
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."
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— Aaron David Miller (@aarondmiller2) August 10, 2017
And Stanford professor Michael McFaul, who served on President Obama's National Security Council and as his ambassador to Russia, tweeted a string of critiques.
Imagine dissing Americans --patriots serving our country under difficult conditions in Russia -to praise Putin. Our president did today. https://t.co/KovoLuMISa— Michael McFaul (@McFaul) August 10, 2017
But former U.S. ambassador to Poland and State Dept. sanctions czar Dan Fried tried to find a different take on Trump's comments.
"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.