Citing the growth of Starbucks and Botox as examples, Ries told a discussion forum in London that the best way to build a brand was to use 'slow-burn PR'.
He added that major ad campaigns should launch only after the brand has already become recognised.
Ries said that PR messages have 'greater credibility' than ad campaigns because of mediation of the message by groups such as journalists.
He acknowledged that using PR as a strategy has its faults, citing lack of control of timing.
Ries was speaking on Wednesday at a forum staged by Edelman. Other speakers included Haymarket editorial director Dominic Mills and Edelman global CEO Richard Edelman.
Mills said he considered Ries's 'faith in the power of PR somewhat misplaced', citing brands such as Direct Line as 'products that have successfully launched with ads'.