"Contacted Trump Jr this morning on why he should publish his emails (i.e with us)," tweeted Mr Assange, who is based at the Ecuadorian Embassy in London. "Two hours later, does it himself."
Mr Trump Jr released the emails on Tuesday after The New York Times told him they had the documents and would be reporting on them. Mr Assange said he told Mr Trump Jr to release the documents "because his enemies have it - so why not the public?"
It was not clear whether Mr Assange's use of the word "enemies" was the reference to the media or political rivals, the report said.
The emails reveal correspondence between Mr Trump Jr and his acquaintance Rob Goldstone as they set up a meeting with Kremlin-linked lawyer Natalia Veselnitskaya, who promised to share incriminating evidence on Ms Clinton to help the Donald Trump campaign.
"This is obviously very high level and sensitive information but is part of Russia and its government's support for Trump," Mr Goldstone wrote in one email.
Mr Trump Jr said that nothing came out of the meeting and that Ms Veselnitskaya only wanted to talk about a US policy on Russian adoptions.
He denied any wrongdoing in taking the meeting, but lawmakers were vying to get him to testify before House and Senate committees.
US President Donald Trump said in a statement that Mr Trump Jr "is a high-quality person and I applaud his transparency".