The Congresswoman said she had resisted calling for the president to undergo investigation, and that she would rather work on other priorities, but that the behaviour detailed in the newly released report had forced her to back calls for his removal.
Ms Ocasio-Cortez said she took "no pleasure" in the discussion, "but the report squarely puts this on our doorstep".
The stance puts her at odds with more senior Democrat colleagues. House speaker Nancy Pelosi last month said impeaching Mr Trump would "divide the country" and that he is "not worth it".
But Ms Ocasio-Cortez claimed the report clearly pointed to wrondoing on the president's part and that Congress must investigate.
"Mueller’s report is clear in pointing to Congress’ responsibility in investigating obstruction of justice by the president," she wrote on Twitter.
"It is our job as outlined in Article 1, Sec 2, Clause 5 of the US Constitution.
"As such, I’ll be signing onto Rashida Tlaib's impeachment resolution."
She acknowledged the proceedings were likely to run into problems with objections from the Senate and elsewhere. But she said she was unable to justify backing the investigation all the same.
"While I understand the political reality of the Senate and election considerations, upon reading this DoJ report, which explicitly names Congress in determining obstruction, I cannot see a reason for us to abdicate from our constitutionally mandated responsibility to investigate."
She wrote on Twitter that she would rather be working on other priotities but that she had no choice but to give it her backing.
"Many know I take no pleasure in discussions of impeachment. I didn’t campaign on it, and rarely discuss it unprompted.
"We all prefer working on our priorities: pushing Medicare for All, tackling student loans, and a Green New Deal.
"But the report squarely puts this on our doorstep."
Fellow new Congresswoman Ilhan Omar also said she had been convinced to back the impeachment of Donald Trump.
"Impeachment is part of our constitutional responsibility," she wrote on Twitter. We have an obligation to investigate whether the President committed impeachable offenses", pointing to "obstruction of justice, violating the Emoluments Clause, collusion, and abuse of power" as possible avenues for investigation.