Riom replaces current UK MD Bruno Fromage, who is stepping down for medical reasons less than a year after being appointed to the role.
Since joining the dairy giant in 2000, Riom has held a number of different positions within the wider Danone group.
He was general manager for biscuits in Danone's Benelux division in 2002, before being promoted to oversee Danone's biscuits operations across Northern Europe in 2004.
Following his stint in biscuits, Riom was then appointed general manager for Danone Dairy in France, in 2007, and subsequently moved to his current position as SVP for people and organisational development at group level.
He has also previously worked for Kraft Foods' US and French operation as well as for P&G in France.
His new role as UK MD, which he starts on 1 September, will be the first time Riom has worked in the UK dairy industry.
He will oversee a portfolio that includes the flagship Activia and Actimel yoghurt brands as well as Shape yoghurts, and joins at a busy time for the business, with high-profile initiatives such as Activia pouring yoghurt and the rebrand of Shape now under way.
Commenting on his new role, Riom said he was "very excited" to be joining Danone's UK operations. "The dairy industry in the UK has immense potential, and I am eagerly looking forward to working with the team in supporting Danone further growth in this market," he said.
Fromage started as UK MD in November last year. He was previously in charge of Danone's dairy operations in Portugal.
Danone has also appointed Corinne Chant as its new UK marketing director, replacing François Rillet, who is moving on to become marketing director for France.
Chant was previously UK marketing manager for Danone's Activia yoghurt brand.