The Northern Ireland international arrived at stadiummk on Friday afternoon to complete the deal, which runs until the end of the 2014/15 campaign, inclusive of the play-offs should the Dons reach them.
Grigg, 23, is well accustomed to Sky Bet League 1 having spent five years with Walsall before moving to Griffin Park in August 2013. The forward, who netted an impressive 20 goals in 46 appearances in his final season with the Saddlers, also helped the Bees secure promotion to the Championship last season.
Speaking to mkdons.com, Dons boss Karl Robinson said: “I’m absolutely over the moon. He’s the one I’ve wanted for the last 12 months.
“When he played against us for Walsall, I thought he was an outstanding talent. He’s a name at this level, he’s a goalscorer at this level and he’s an Northern Ireland international too. He’s got the pedigree to come in and do well.
“He’s played in a similar system for Walsall and for Brentford, so he’s used to the way we play. I don’t think he’ll need much integration.
Robinson continued: “That’s now two natural-born goal-scorers I’ve been able to bring in after Tom and in my ideal world the third one would be a big man so I can change the way we play.”
Born in Solihull, Grigg joined Birmingham City at the age of seven and progressed through the youth ranks at St. Andrews before signing non-contract terms with non-league outfit Stratford Town in September 2007.
Grigg drew interest from a host of professional sides during his time with the Town, eventually earning himself a scholarship with Walsall in the summer of 2008.
Making his league debut for the Saddlers in December of the same year, Grigg had to wait until the 2010/11 campaign for his opportunity to establish himself a first-team regular, which he did by scoring four goals in 30 appearances despite only being 19.
The following campaign, Grigg, again, netted four times and in doing so earned himself a call-up to the senior Northern Ireland side for a friendly with Holland. He started the match against the likes of Robin Van Persie, Arjen Robben and Wesley Sneijder and was one of five to play the full ninety minutes.
However, it was in 2012/13 when Grigg really came to the fore, netting seven goals prior to the New Year including a fine brace against the Dons on Boxing Day. His form continued into 2013, when he netted his first career hat-trick in a victory over Carlisle United in February.
Grigg finished the season with a run of 10 goals in 11 games – form which earned him Walsall’s Player of the Season and Players’ Player of the Season awards and saw him linked with clubs such as Derby County, Aston Villa and Southampton.
In July 2013, Grigg signed a three-year deal with Brentford for a tribunal fee of £325,000, making his debut for the Bees in a 1-1 draw away at Port Vale before scoring a brace the following weekend against Sheffield United.
Despite injuries, international call-ups and being played out of position last season, Grigg still managed to score five times in just 16 starts as he helped Mark Warburton’s men finish second in League 1 and secure promotion to the Championship.
Grigg will feature in tomorrow’s friendly with Aylesbury FC which kicks-off at 1pm at Haywood Way.