May has embarked on a nationwide campaign to sell sceptical Britons on the EU divorce arrangement she secured in Brussels last weekend.
She will spend part of Wednesday in Scotland, whose small nationalist party in the British Parliament rejects the agreement, and where fishing is a particularly emotive issue.
Lawmakers will debate May's Brexit deal on December 11, with early vote counts stacking up against the British premier.
"At long last, we will be 'an independent coastal state' again -- taking back full sovereign control over our waters, and free to decide for ourselves who we allow to fish in our waters," May will say in an address near Glasgow, extracts of which were released by Downing Street.
"The EU maintained throughout the negotiation process that it wanted to link overall access to markets to access to fisheries. It failed in the withdrawal agreement and it failed again in the political declaration," May will say.
"I have been robust in defending the interests of Scottish fisherman so far -- and I will always be so." May had promised to make sure Britain leaves the EU's Common Fisheries Policy after Brexit enters into force on March 29.
But under the deal sealed on Sunday, that will only happen after a transition period scheduled to end in December 2020.
Scottish fishermen are furious they will have to continue obeying EU rules during these 21 months, including much-despised quotas and allowing European vessels access to UK waters.