Justice Minister Rob Nicholson announced the move on June 17 in Toronto, joined by the heads of Canadian groups that fight child abuse online and elsewhere.
Nicholson said no country can fight the crime alone, and said he looked forward to working with other countries in the Global Alliance Against Child Sexual Abuse, which was formed in December by the United States and more than 40 other countries.
The head of the Canadian Centre for Abuse Awareness strongly endorsed Canada's membership at the news conference.
Ellen Campbell called it a good step forward in bringing offenders to justice.
But a briefing note to Nicholson in February called membership in the alliance "symbolic" and "non-binding."
"Membership in the Global Alliance will not require onerous reporting or resources," says the document, obtained by The Canadian Press under the Access to Information Act.
"Indeed, membership is symbolic and there will be no obligations placed on Canada or any other member state."
Canada was invited to join the alliance last July by officials in the United States and the European Union.
The federal government declined to accept immediately, though it sent two observers to Brussels on Dec. 5 for the launch of the new organization.
"Canada made it clear that it attended the December 5th meeting as an observer and did not commit to joining the Global Alliance at that time," says the Feb. 6 briefing note.
At the meeting, member countries promised to commit to policy targets and goals.
Nicholson told the Toronto news conference that Canada's new membership "will formalize with a whole new range of countries ... the intention to share information and to co-operate with each other in these investigations."
The minister also used the occasion to review the Conservative government's legislation since 2006 to protect children.
A spokeswoman for Nicholson defended Canada's membership, noting that common policy targets include enhanced efforts to identify victims, and providing them support and protection.
"This is an international crime, requiring an international response," Julie Di Mambro said in an email.
"Canada must continue to work together with like-minded countries to combat this egregious crime."