LF15, nicknamed "Lassie", touched down on Thursday afternoon to join her mate LM12, who arrived on 17 March.
The charity said Lassie began rearranging her nest before the birds made their first attempts at mating.
The pair, who can be watched on the reserve's live webcam, fledged three chicks in both 2015 and 2016.
Charlotte Fleming, Perthshire ranger for the Scottish Wildlife Trust, said: "We knew it was LF15 as soon as she arrived.
"She has unmistakable features including a prominent dark brown "Y" on the top of her head and quickly set to work rearranging her nest to her satisfaction."
Ms Fleming said LM12, who has spent the last week fending off crows and creating a deep cup on the nest to hold this season's eggs, joined his mate within an hour.
She said: "While it was a frosty reception with lots of shouting and mantling at first, it only took a matter of minutes for them to begin to mate."
A female osprey has reunited with her mate at the Scottish Wildlife Trust's Loch of the Lowes reserve in Perthshire.