ABSTRACT Legionella spp. cause the severe pneumonia Legionnaires' disease. The environmental bacteria replicate intracellularly in free-living amoebae and human alveolar macrophages within a distinct, endoplasmic reticulum (ER)-derived compartment termed the Legionella -containing vacuole (LCV). LCV formation requires the bacterial Icm/Dot type IV secretion system (T4SS) that translocates into host cells a plethora of different “effector” proteins, some of which anchor to the pathogen vacuole by binding to phosphoinositide (PI) lipids. Here, we identified by unbiased pulldown assays in Legionella longbeachae lysates a 111-kDa SidC homologue as the major phosphatidylinositol 4-phosphate [PtdIns(4) P ]-binding protein. The PI-binding domain was mapped to a 20-kDa P4C [ P tdIns( 4 ) P binding of Sid C ] fragment. Isothermal titration calorimetry revealed that SidC of L. longbeachae (SidC _Llo ) binds PtdIns(4) P with a K _d (dissociation constant) of 71 nM, which is 3 to 4 times lower than that of the SidC orthologue of Legionella pneumophila (SidC _Lpn ). Upon infection of RAW 264.7 macrophages with L. longbeachae , endogenous SidC _Llo or ectopically produced SidC _Lpn localized in an Icm/Dot-dependent manner to the PtdIns(4) P -positive LCVs. An L. longbeachae Δ sidC deletion mutant was impaired for calnexin recruitment to LCVs in Dictyostelium discoideum amoebae and outcompeted by wild-type bacteria in Acanthamoeba castellanii . Calnexin recruitment was restored by SidC _Llo or its orthologues SidC _Lpn and SdcA _Lpn . Conversely, calnexin recruitment was restored by SidC _Llo in L. pneumophila lacking sidC and sdcA . Together, biochemical, genetic, and cell biological data indicate that SidC _Llo is an L. longbeachae effector that binds through a P4C domain with high affinity to PtdIns(4) P on LCVs, promotes ER recruitment to the LCV, and thus plays a role in pathogen-host interactions.