Lithuania's average annual temperature hit 9.2 degrees Celsius last year, 2.3 degrees above the multi-annual average, it said.
The 2020 temperature beat previous highs set in 2019 and 2015, at 8.8 and 8.3 degrees, respectively.
2020 marks the first year on record that the average annual temperature in Lithuania has crossed 9 degrees.
Last year also saw the warmest winter and autumn seasons on record.
"2020 was the first year in Lithuania when the average temperature of all 12 months was positive," the service said. "A distinctive feature of 2020 was a very low number of snow-covered days and the absence of cold days."
The first regular meteorological observations in Lithuania began in Vilnius back in 1778. Daily weather measurements in the capital have been taken for almost 250 years.
The average annual amount of precipitation in Lithuania in 2020 was 646 mm, 7 percent below the multi-annual norm of 694 mm.