The dieting brand will task Davidson with heading up brand strategy, which will include the development of integrated marketing campaigns.
Weight Watchers recruited Davidson after Reisser-Weston left in order to become global brand director at Castrol.
Davidson will be supported in her role by advertising brand manager Melissa Matthews, who was previously head of brand strategy at Samsung.
Weight Watchers said: "Julie brings a wealth of experience to the Weight Watchers brand from her previous marketing director roles at Levi Strauss, L’Oreal UK, Lasminute.com and the Boots beauty brands, to continue to drive us forward as industry leader."
Follow Matthew Chapman at @mattchapmanUK