Presidential secretary Diego Canepa said today that Mr Mujica signed it last night, the last formal step before the law could take effect.
Bureaucrats now have until April 9 to write the fine print for regulating every aspect of the marijuana market, from growing to selling in a network of pharmacies.
They hope to have the whole system in place by the middle of next year.
But as of today, growing pot at home is legal in Uruguay, up to six plants per family and an annual harvest of 480 grams.
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