Amyotrophic lateral sclerosis (ALS) and behavioral variant frontotemporal dementia (bvFTD) lie on a clinical, pathologic, and genetic continuum. Neuroimaging techniques have proven to be potentially useful to unravel the shared features of these syndromes. Using resting-state functional magnetic resonance imaging (RS-fMRI), we investigated functional connectivity of brain networks in 15 ALS and 15 bvFTD patients in early stages of disease and 15 healthy controls, looking expressly for connectivity pattern divergence or overlap between the 2 disorders. Compared with controls, we found decreased RS-fMRI signals within sensorimotor, right frontoparietal, salience, and executive networks in both patient groups. Within the default mode network (DMN), divergent connectivity patterns were observed, with RS-fMRI signals in the posterior cingulate cortex enhanced in bvFTD patients and suppressed in ALS patients. Our findings confirm that ALS and bvFTD not only broadly share common RS-fMRI connectivity patterns, probably representing different phenotypical expressions of the same neurodegenerative process, but also differ in the DMN, probably reflecting a different stage of neurodegeneration.