Rainier III ( Rainier Louis Henri Maxence Bertrand Grimaldi ; 31 May 1923 -- 6 April 2005 ) ruled the Principality of Monaco for almost 56 years , making him one of the longest ruling monarchs in European history .
Rainier Louis Henri Maxence Bertrand Grimaldi ( 31 May 1923 -- 6 April 2005 ) , also known as Rainier III , ruled the Principality of Monaco for almost 56 years . He was one of the longest ruling monarchs in European history .