Speaker Gene Zwozdesky sent a letter to both the Wildrose and Liberal parties Tuesday confirming his decision.
Zwozdesky said incumbency was the key factor in his ruling.
"The long and short of this decision is I reviewed all the authorities I could possibly review, as well as all the precedents I could possibly review in similar circumstances," Zwozdesky said later in a phone interview.
"I concluded the overwhelming evidence favoured holding on to the incumbent, because the incumbency factor is really the key thing here when a tie exists."
A defection last week by former Wildrose leader Danielle Smith and eight other caucus members had left both the Wildrose and Liberals with five seats each.
The Liberals quickly announced they wanted the Speaker to install them as the new Official Opposition.
The Wildrose named Heather Forsyth interim leader Monday and immediately contacted Zwozdesky about the appointment.
"What they said was there was a party with a leader that was the Official Opposition. And without having a leader, it might have been a different question," said Zwozdesky.
"They were quick to choose a leader all on their own and apprised me of that. It was taken into consideration."
Official Opposition status gives a party more money and the right to ask more questions in the legislature.
Wildrose house leader Shayne Saskiw applauded the decision.
"We want to thank the Speaker for his ruling affirming the Wildrose as Alberta's Official Opposition," he said.
"Given prior precedent, it is our belief this ruling strengthens our democracy. In the last election, over 440,000 Albertans overwhelmingly voted for the Wildrose to be their voice in the legislature, a strong mandate neither the third nor the fourth parties received."
Liberal Leader Raj Sherman said he was disappointed.
"We have always maintained that two conservative parties opposing each other is bad for democratic dialogue," he said.
"In light of that, the Speaker's decision to recognize the Wildrose as the Official Opposition is disappointing. However, this is not the end."
With last week's defections, the Tories hold 72 of 87 legislature seats. The NDP has four seats and there is one Independent.
— By Bill Graveland in Calgary
Follow @BillGraveland on Twitter
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