The 22-year-old impressed at the recent U23 Giro d’Italia, where he won a mountain stage, spent a day in the race leader's pink jersey and finished fifth overall.
Williams will ride as a stagiaire with Bahrain-Merida from August 1 and be a neo-pro with the team in 2019. The Welshman raced with the JLT Condor team managed by John Herety in 2016 and joined the SEG Racing Academy for 2017.
SEG started as a rider agency and works with Wout Poels, Dan Martin, Niki Terpstra, Sep Vanmarcke and Bauke Mollema. Alumni of the SEG Racing Academy include Fabio Jakobsen, who joined Quick-Step Floors for 2018, and Koen Bouwman, who won a stage at the 2017 Critérium du Dauphiné.
Williams won the Rode de L'Isard stage race in the Pyrenees in early May before his impressive ride at the Baby Giro.
He is an aggressive climber who knows how to win races and is already being compared to UAE Team Emirates' Irishman Dan Martin.
"It feels like everything has kind of come together over the last two months with the Isard in France and then the Baby Giro," Williams recently told the Velo UK website. "To see some results come in is good for morale."