Mr Alexander, 28, was killed by an Improvised Explosive Device, after having won the Military Cross for charging a Taliban gunpost to provide cover for a wounded comrade. He and his family lived in Plymouth, the Marines being based there, and they along with others will be travelling to the ceremony this morning.
It is only the second such plaque to be fixed to the bridge, the first having been in honour of another soldier who in 1919 died attempting to rescue someone from the water.
Marine Alexander's mother told the Plymouth Herald:
"It is a tremendous solace to Sam's family and friends to have this memorial to him in so fitting a place.
"His indomitable spirit, love of adventure and courage were evident at a very young age; as was his loving nature.
"It is the wanting to make a better life for others that took him to Afghanistan, where he sincerely believed in the job that he was doing.
"I am incredibly proud of him, but still miss him terribly."The mayor of Hammersmith and Fulham, Councillor Frances Stainton, added:
"Sam Alexander has become a name to conjure with.
"He stands for all that is best in our young men. He stands for the courage that goes way beyond what is asked for and for that unique love and loyalty soldiers in combat under fire discover."