The whale, spotted in the sea on August 31, was seen floating at the top of the water.
Dorset Wildlife Trust are asking people to contact them if they find the whale washed ashore.
Baleen whales range from six metres to 34 metres.
When a whale dies in the sea, its carcass usually eventually sinks to the bottom and can become home to a new ecosystem as other fish and sea animals eat off the meat.
However, sometimes whales are known to wash ashore.
It comes after a spotting of what the Dorset Wildlife Trust believe could have been a minke whale last week on August 23.
© Dorset Wildlife Trust
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