Trump's remarks came after The New York Times reported that McGahn had "co-operated extensively" with the investigation by special counsel Robert Mueller, taking part in at least three interviews with investigators totaling 30 hours.
"I allowed White House Counsel Don McGahn, and all other requested members of the White House Staff, to fully co-operate with the Special Counsel," Trump tweeted.
"In addition we readily gave over one million pages of documents. Most transparent in history. No Collusion, No Obstruction. Witch Hunt!"
Trump views the investigation headed by special counsel Robert Mueller as a stain on his presidency, and has repeatedly pushed for it to be ended.
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The Times reported that McGahn had provided "detailed accounts" about episodes relating to whether Trump sought to obstruct justice, citing a dozen current and former officials and other sources.
The information McGahn provided included the president's attempts to fire the special counsel, and Trump's comments and actions during his firing of FBI director James Comey, according to the Times.
The newspaper said it was unclear whether Trump knew the extent of McGahn's cooperation, noting that it is rare for a lawyer be so open with investigators.
