The comments by Bob Corker, chairman of the US Senate Foreign Relations Committee, came as tensions in the region continue over the crisis, with US Secretary of State Rex Tillerson expected to make an unscheduled stop in Doha on Thursday.
"The amount of support for terrorism by Saudi Arabia dwarfs what Qatar is doing," Corker said on Wednesday.
Saudi Arabia, the United Arab Emirates, Bahrain and Egypt cut ties with Qatar on June 5 and imposed a land, air and sea blockade on the country.
The quartet accuse Qatar of funding "terrorism", an accusation Qatar rejects as "baseless".
Corker said he was "really disappointed to see what Saudi Arabia did after having a great summit and bringing everybody together", referring to May's Riyadh conference that saw leaders of the GCC and other Arab states meet with US President Donald Trump.
"I think this is quite possibly a rookie mistake by a crown prince who I think could be the future for Saudi Arabia," Corker added.
The Tennessee senator made the comments while addressing the former US ambassador to Israel during a legislative committee hearing.
Al Jazeera's Heidi Zhou-Castro, reporting from Washington, DC, said the Saudi crown prince is "widely seen as among the blockade's chief architects and a chief organiser of the alliance behind it".
She said that Corker has been among the blockade's strongest US critics, adding that the senator has threatened to block weapons sales to members of the GCC until there was a clear path for settling the dispute.
Tilllerson's Gulf visit
Corker's comments came as Tillerson wrapped up the third leg of his four-day trip to the Gulf aimed at helping solve the GCC crisis.
Having shuttled between Kuwait, Qatar and Saudi Arabia in an effort to help solve the GCC crisis, Tillerson is set to make an unexpected return to Doha on Thursday before returning to the US.
On Wednesday, Tillerson met the Saudi king, crown prince and his counterparts from the Saudi-led quartet.
The Jeddah visit followed meetings in Doha with the Qatari emir and foreign minister on Tuesday.
Tillerson told reporters that the Qatari government had "reasonable" views during the crisis.
"I think Qatar has been quite clear in its positions, and I think those have been very reasonable," he said.
Foreign Minister Sheikh Mohammed bin Abdulrahman Al Thani said the memorandum of understanding was not related to the current dispute.