Bophud station police investigator Pol Maj Udomsak Thappa said the police were alerted of the death of Ms Thies Saskia by Bangkok Samui hospital around midnight.
She suffered wounds on both legs, stomach, back and arms, the officer said.
Her friend identified as Jiovana Rassi, also a German tourist, received slight injury as she swam to rescue her friend after hearing she screamed while swimming in the sea in front of Lamai beach.
She is still at hospital for monitoring of the wound.
Investigation later revealed that both tourists checked in at a bungalow on Lamai beach.
Shortly later Bungalow employees heard loud scream from the beach, they rushed to the source of the sound and saw both tourists in the sea.
They cried in pains with wounds believed to be stung by box jellyfishes on their bodies.
They were given primary care by rescue workers of Samui municipal rescue centre with vinegar to relieve their pains, and later rushed to Bangkok Samui hospital.
But Ms Thies Saskia died later from the deadly poisons.
The body of the dead tourist will be flown to Police General Hospital in Bangkok this morning for autopsy.
Jet Ski club president Withaya Suksom, said he was alerted of a large number of box jellyfishes in the sea of Lamai beach by residents and club members yesterday.
He immediately coordinated with the marine police and also asked jet skiers to go out warning tourists swimming in the water to stay off the sea.
Besides he also sought cooperation from all bungalow and hotel operators to put up warning signs on the beach and advise tourists not to swim in the sea during this period.
He said he never expected the incident would happen at night.
The box jellyfish is one of the world’s most toxic animals and its stings usually cause victims to die quickly and in agony.
A Thai woman died on Koh Phangan from box jellyfish stings in August this year.
Experts say that the only treatment for box jellyfish is common vinegar. The vinegar does nothing to dull the pain but it does combat the toxicity. Without vinegar, the jellyfish stings continue to attack and destroy the victim’s flesh.