Rachel Reeves said she is "angry" that US President Donald Trump chose to go to war with Iran, adding there is no "clear" exit strategy.
The chancellor said the decision is "causing real hardship for people now", with higher inflation, weaker economic growth and lower tax receipts among the potential consequences.
Relations between the US and the UK have become increasingly strained following Prime Minister Sir Keir Starmer's refusal to be drawn further into the war with Iran.
The direct criticism of Trump by Reeves went further than Sir Keir, who earlier on Wednesday said he would not change his decision "whatever the pressure".
Trump has repeatedly criticised Sir Keir in recent weeks after the PM refused to allow the US to use UK bases for its initial offensive strikes against Iran.
