The Greek broadcaster was closed down in 2013 under austerity measures.
The left-wing Syriza party made the reinstatement of ERT a key pledge in January’s election, which it won.
Syriza said all of the more than 2,600 employees made redundant in 2013 have been offered jobs by the station.
Presenter Nikos Aggelidis said at the start of the first show on June 11: “It’s a special day for all Greeks, for those who love Greece and for those who love freedom of information.”
“We’re nervous. We’re very touched.”
His co-host Vasiliki Haina said: “It’s a special day for us, a difficult day.”
ERT’s television channel went off air in the middle of a program in 2013, and viewers saw only a black screen. There had been no warning of the channel coming to an end.
The decision to end ERT, which cost €300 million ($337 million) to run, led to protests on Greek streets.
Some said the network was plagued by cronyism and had appeared immune from cuts and reform.
While ERT continued to broadcast on the internet, a smaller-scale replacement, Nerit, was introduced in May 2014.
Nerit enjoyed moderate success despite a smaller staff, and secured Champions League football coverage and rights to broadcast the Eurovision song contest.
Nerit’s television operation merged with ERT on June 11.
The letters N and I were removed from the sign on Nerit’s building to spell ERT earlier this week.
ERT will be funded by a €3 surcharge added to electricity bills – less than the surcharge before it went off air.
Greece’s government, which is fighting to secure loans to help stabilize the country’s finances, called the reinstatement of ERT “a victory for democracy”.
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