A modular free space optical (FSO) communications device may include an adaptive optics (AO) module and an interchangeable optical relay (OR) module. More particularly, the AO module may include an AO housing and at least one AO device carried thereby. Also, the interchangeable OR module may include an OR housing and at least one OR device carried thereby. The at least one OR device of a given OR module may provide a predetermined focal length range from among different focal length ranges for respective OR modules. In addition, the OR housing and the AO housing may be interchangeably connectable and establish an optical path between the at least one OR device and the at least one AO device when connected.