The former couple ended their 14-month marriage in December when Russell filed for divorce citing "irreconcilable differences" - but Katy, 27, has reportedly had a change of heart.
A source said: "Katy's been trying to get him back! She's a lost soul right now."
Russell, 36, has since moved on with model Nikolett Barabas, while Katy has been linked to Chanel star Baptiste Giabiconi. But according to US Weekly, a recent trip to India - where she tied the knot with the Arthur actor in October 2010 - made her realise what she had lost.
The insider said: "She'd already been missing Russell. Being back there stirred up even more old feelings."
However, a reunion may be a long way off with the pair still, "trying to get to the point where they can at least be friends".
Russell - who waived his right to a chunk of Katy's wealth in the divorce settlement, which is set to be made official in June - has previously insisted he is already over the split.
He said: "I'm doing well. You don't need to brush it off if you don't get it on you in the first place.
"I don't have to let anything go. I don't hold on to anything negative. It's the same as zero-ness.
"It's not about letting it go. You can't let go of something you don't hold, you know. It's meaningless, [and] much of our world is predicated on meaninglessness. And I don't do that anymore."
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