The pair met while filming a TV adaption of Wuthering Heights last year (09) and their friendship quickly blossomed into romance.
Riley showed off a diamond on her ring finger at Thursday's (08Jul10) premiere of Hardy's new movie, Inception - and reports suggest they're planning to exchange vows next year (11).
A source tells Britain's Mail on Sunday newspaper, "As soon as they did their first scene they realised that they were attracted to one another and things just developed.
"They had to keep their courtship very under the radar and they are so relieved now that they can just be open about their romance.
"Tom and Charlotte are so happy together and they just announced recently that they are getting engaged. They are perfectly matched and deeply in love with one another."