The decision came a day after Russia fired at and captured three Ukrainian vessels, triggering a sharp escalation in tensions between the two countries.
Parliament was expected to debate the decree later on Monday though a televised session of a parliamentary committee unanimously supported it.
Ukrainian legislation on martial law allows for a slew of curbs, including restrictions on movement and peaceful assembly, curfews, and restricting the media, though Poroshenko's decree made no specific references to such restrictions.
Eleven measures outlined in the decree provide for mobilizing reserve forces, organizing the air defence of important state facilities and taking urgent steps to bring in enhanced cyber security measures and ensure public order.
There was no mention of a presidential election due to be held in March. Poroshenko's move has prompted speculation by critics and opposition politicians that he might use it to delay the vote.
A twelfth provision of the decree was not published.
(Reporting by Natalia Zinets; writing by Matthias Williams Editing by Richard Balmforth)
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