The reading beats the previous record on the earth's southernmost continent of 17.5C (63.5F) in March 2015 by 0.8C, according to the Argentine station Esperanza, which collected the data.
The tweet reporting the news from Argentina's meteorological association was shared by the United Nations's World Meteorological Organisation (WMO).
The Antarctic Peninsula, the northwest tip near South America, is among the fastest warming regions on earth, with temperatures rising almost 3C (5.4F) over the past 50 years, the WMO said.
Some 87% of the glaciers along its west coast have "retreated" over those five decades and have shown an "accelerated retreat" in the past 12 years, it added.
Professor James Renwick, a climate scientist at Victoria University of Wellington, told the Guardian Australia the WMO committee would likely reconvene to ratify the new record.
He told the paper: "The reading is impressive as it's only five years since the previous record was set and this is almost one degree centigrade higher. It's a sign of the warming that has been happening there that's much faster than the global average.
"To have a new record set that quickly is surprising but who knows how long that will last? Possibly not that long at all."
Esperanza, located near the northern tip of the Peninsula, has been collecting data since 1961.
The reading breaks the 2015 record for the Antarctic continent, defined as the main continental landmass and adjoining islands by the WMO.
The record for the Antarctic region - defined as all land and ice south of 60 degrees latitude - is 19.8C (67.64F) recorded on Signy Island in January 1982, the WMO said.