"Julian Assange said 'a 14 year old could have hacked Podesta' -- why was DNC so careless? Also said Russians did not give him the info!" the Republican posted on his preferred communication platform, Twitter.
Trump was referring to thousands of emails hacked from the Democratic National Committee and from Clinton campaign chief John Podesta, which were published by WikiLeaks in the weeks ahead of the November 8 presidential election.
"Somebody hacked the DNC but why did they not have 'hacking defense' like the RNC has," Trump added, referring to the Republican National Committee.
The US intelligence community has concluded that the hack-and-release of the emails was designed to put Trump -- a political neophyte who has praised Russian leader Vladimir Putin -- into the Oval Office.
Moscow has repeatedly dismissed the allegations that it was responsible for the cyber-meddling.
Assange said in an interview with Fox television broadcast Tuesday that Podesta's Gmail account was "something a 14-year-old kid could have hacked."
He insisted that no Russian government-linked party was the source of the hacked material.
"The source is not the Russian government. It is not state parties," the 45-year-old Australian told Fox.
Trump has asserted that US intelligence services were mistaken when they said Iraq had weapons of mass destruction, a finding that led the country into war, and has publicly and repeatedly questioned their work.