The Emmerdale actor began dating Porter when she joined the cast of the ITV1 soap as Roz Fielding. However, they split in March, with Porter said to have tweeted that it was "due to his unreasonable behaviour and disloyalty".
The 22-year-old has now agreed to take Miller - who plays Aaron Livesy in the drama - back, The Sun claims, as pictures of the pair enjoying a break in the Lake District were revealed.
"When they split Kirsty refused to speak to Danny," a source told the paper. "But he was determined to win her back and now their relationship is back on track.
"They've been seeing each other again for a couple of weeks and they can't keep their hands off each other."
The insider added: "It's as if they've fallen in love all over again."
In April it was announced that Porter will leave her role as Roz at the end of her current contract.
> Danny Miller 'may get back with Kirsty-Leigh Porter'