Saudi shut its side of Qatar's only land border in June 2017 as part of a package of sanctions it said was a response to Doha's alleged backing of "radical Islamist groups" and "closeness ".
Qatar always denied the charges.
"Yes, the border is open," said one Qatari source, while another confirmed that traffic at the Abu Samrah crossing, 120 kilometres (75 miles) south of Doha, resumed around 0700 GMT.
Saudi Arabia, along with the , Bahrain and Egypt which also imposed and trade, agreed to lift the restrictions at a Gulf Cooperation Council summit in the kingdom on Tuesday.
Read also:
On January 4, Kuwaiti Foreign Minister Ahmad Nasser Al-Sabah announced on state television that a deal had been agreed to "open the airspace and land and sea borders between the Kingdom of Saudi Arabia and the State of Qatar".
"They are letting them go from the Qatar side," said a witness at the border on Saturday.
Qatar has announced strict coronavirus control measures for those arriving from the Saudi side that will require travellers to present a negative test result, undergo a fresh test at the frontier, and quarantine in a government-approved hotel for one week.
"It is a great joy, I bought this new car, a Land Cruiser, in order to go and celebrate with my relatives in Saudi Arabia, and I took the coronavirus test and waited here hoping they would allow us to cross at any moment," said Zaid Muhammad al-Marri, 23, a Qatari whose mother is Saudi, ahead of the border reopening.
Follow us on , and to stay connected