The American striker was due to remain at the County Ground until the end of the season having arrived from Elland Road in late February, a capture made by former boss Danny Wilson.
Grella`s time under Wilson lasted just one match before he parted ways with the Robins, although the 24-year-old initially remained in favour under interim boss Paul Hart - starting each of his first five games.
But one substitute appearance in the next two matches was enough to leave Grella bemoaning a lack of first-team opportunities and he came to an agreement with Hart to return to Leeds before Saturday`s 1-0 win at Brentford, a match in which he took no part.
Speaking after the much-needed weekend victory, Hart said: "He (Grella) came in to see me on Friday and said he would prefer to go back to Leeds, so I let him go. We don't need people who don't want to be here."
Meanwhile, Grella used Twitter to say: "I've decided to come back to Leeds because under the new management I wasn't getting games. My reason for joining Swindon was to help them and to also get games in...I want to say thanks to the Swindon fans for everything and know that I have a great respect for the club and all the people involved. I hope they stay up because they have a great group of lads and the club deserves to be up higher."
Source: PA