R v Big M Drug Mart Ltd[2] (Her Majesty The Queen in Right of Canada v Big M Drug Mart Ltd) is a landmark decision by Supreme Court of Canada where the Court struck down the federal Lord's Day Act for violating section 2 of the Canadian Charter of Rights and Freedoms.

On Sunday, May 30, 1982, the Calgary store Big M Drug Mart was charged with unlawfully carrying on the sale of goods on a Sunday contrary to the Lord's Day Act of 1906.

The Supreme Court ruled that the statute was an unconstitutional violation of section 2 of the Canadian Charter of Rights and Freedoms, deciding that there was no true secular basis for the legislation and its only purpose was, in effect, to establish a state religious-based requirement, and was therefore invalid.

[3] In that case, Chief Justice Brian Dickson wrote that this freedom at least includes freedom of religious speech, including "the right to entertain such religious beliefs as a person chooses, the right to declare religious beliefs openly and without fear of hindrance or reprisal, and the right to manifest religious belief by worship and practice or by teaching and dissemination."

The Lord's Day Act was the first law in Charter jurisprudence to be struck down in its entirety, and some of the section 1 analysis in the decision played a role in developing the "Oakes test" in the later case R v Oakes.