The Joint Medical Command is responsible for providing healthcare to all three armed services - the Royal Navy, Army and Royal Air Force.
This stretches from training frontline emergency medical services through to the care and rehabilitation of the seriously wounded.
The three-year contract was awarded following a competitive tendering process.
JMC head of corporate communications Roger Eustis said: 'We are experiencing high levels of operational activity and JMC is also restructuring to deliver the best possible medical care to all serving personnel.
'Effective and timely communications to our own people are a vital part of our operation. Lexicon has shown it is consistently on target in meeting our requirements.'
Lexicon will provide electronic communications for JMC, including electronic internal newsletters covering clinical and news developments, and the maintenance of a password-protected news and information website.
The JMC, which was formed in April 2008, forms part of the Surgeon General's overall responsibilities.