The codeshare agreement came into force for flights on the Maputo-Beira route on Wednesday and would be followed in March with a similar arrangement taking effect for flights on the Maputo-Tete-Quelimane route, according to the airlines.
The agreement means that LAM passengers would be able to travel on Fastjet flights and vice versa.
LAM signed a Memorandum of Understanding with Fastjet in March 2018 under which the two airlines committed to long-term cooperation involving codeshare and route agreements, as well as collaboration in other commercial activities such as cargo, engineering and maintenance.
The collaboration is seen by aviation experts as a response to the increased competition due to Ethiopian Airlines entering the market in December.
Ethiopian Mozambique Airlines now flies scheduled flights between the capital city Maputo and Nampula, Tete, Beira, Quelimane, and Pemba.