Vidmantas Urbonas set out Sunday from the resort of Pavilosta in neighbouring Latvia, aiming to swim to the Swedish island of Gotland and thus complete the final 145-kilometre (90-mile) leg of his abandoned attempt albeit back-to-front.
“I start today and shall swim day and night if the weather is friendly,” Urbonas told Lithuanian public radio before setting out.
He declined to speculate how long it could take him to reach Gotland, noting it is difficult to forecast the weather in the Baltic Sea.
Urbonas, 49, who was world triathlon champion in 1998, began his first attempt on July 22. He set out from Kalmar in Sweden, planning to make the 209-kilometre (130 mile) crossing in three stages, with the longest stretch from Gotland to Pavilosta.
He reached Gotland after four days and then pressed onwards, but on July 27, with around 100 kilometres still to go, he was forced to board his support boat after spending hours battling storm waves.
Urbonas had planned to resume the swim when the storm ended, but when the
weather conditions failed to improve he decided a day later to formally call of the attempt.
However, he pledged to cover the missing kilometres whenever possible.
Urbonas, a seasoned activist, is trying to highlight concerns about the heavily trafficked Baltic Sea, which has a very slow rate of natural cleansing.
The Baltic is shallow and virtually closed, with only a narrow outlet across the Straits of Öresund between Sweden and Denmark.
Urbonas has previously swum 460 kilometres down the Nemunas River in Lithuania as part of an environmental campaign.
Last year he also tried to swim the English Channel, but had to halt the attempt due to rough weather.