As a result of the move, Bright's co-founder and creative director Chris Martin has left the agency. Martin was not contactable as the DM Bulletin went to press.
Sean Harris, managing partner of Alphabet, will run the merged agency. "Alphabet Group has been looking to grow over 2009 and it was good to have met a like-minded company that we could bring into the group."
Staff from Bright, including CEO Chris Rendel and planning director David Brown, relocated this week to Alphabet's Old Street office, bringing the agency's total staff tally to 20.
Alphabet's copywriter Simon Tuke and art director Jason King continue to run Alphabet's creative output.
It is not known the extent to which Bright's chairman Sir David Bell, chairman of the Financial Times and Sadler's Wells, will remain involved with the agency.
Bright, which was launched in 2002 by former Bates 141 directors, itself acquired an agency last year. It bought ad agency the Windmill Partnership from founder Chris Rendel for an undisclosed sum.
Alphabet clients include PricewaterhouseCoopers, EA Games and B2B insurer QBE. Bright's roster includes Roehampton University, genealogy website Ancestry.co.uk and the consumer credit side of Experian.
"We were looking to join forces with an above-the-line agency, said Bright account director Elizabeth McCallum. "There will be opportunities for Bright and Alphabet to cross-sell, as Alphabet has clients that could do with some DM, and vice versa."
A decision on whether to retain the Bright name will be taken soon, McCallum said.