GPR18 is a cannabinoid-activated orphan G protein-coupled receptor (GPCR) that is selectively expressed on immune cells. Despite its significant potential as a drug target for inflammatory diseases and cancer immunotherapy, only very few GPR18 ligands have been described to date. In the present study we investigated the structure-activity relationships (SARs) of (Z)-2-(3-(4-chlorobenzyloxy)benzylidene)-6,7-dihydro-2H-imidazo[2,1-b][1,3]thiazin-3(5H)-one (PSB-CB5, 5), the most potent GPR18 antagonist described to date. Analogs were synthesized that exhibit broad modifications of the heterocyclic core and/or variation of substituents at the benzylidene moiety. The compounds were investigated in β-arrestin recruitment assays as inhibitors of human GPR18 activation by tetrahydrocannabinol (THC). Selectivity was assessed versus the cannabinoid receptors (CB1 and CB2) and versus GPR55, another orphan GPCR that interacts with cannabinoids. Phenyloxyalkyloxy-substituted benzylidenethiazinones with long alkyl chains (optimal length: hexamethylene) efficiently blocked GPR18 with similarly high potency as lead structure 5. (Z)-2-(3-(6-(4-Chlorophenoxy)hexyloxy)benzylidene)-6,7-dihydro-2H-imidazo[2,1-b][1,3]thiazin-3(5H)-one (PSB-CB-27, 23) exhibited the best profile: it displayed an IC50 value of 650 nM at GPR18 and showed improved selectivity versus CB receptors as compared to lead structure 5. Importantly, in contrast to 5, which showed only partial inhibition (60%), 23 led to a complete blockade of THC-induced GPR18 activation and is thus a superior tool for target validation. In addition, several compounds, e.g. 18 and 22, were identified as dual GPR18/GPR55 antagonists with similar potency at both targets, and selectivity versus CB receptors.