Hereditary Diffuse Gastric Cancer (HDGC) is a cancer predisposing syndrome mainly caused by germline inactivating variants in CDH1, encoding E-cadherin. Early-onset diffuse gastric cancer (DGC) and/or invasive lobular breast cancer (LBC) are the main phenotypes in CDH1-associated HDGC. CTNNA1, encoding for α-E-catenin, and E-cadherin-partner in the adherens junction complex, has been recently classified as a HDGC predisposing gene. Nevertheless, little is known about CTNNA1 tumor spectrum in variant carriers and variant-type associated causality. Herein, we systematically reviewed the literature searching for CTNNA1 germline variants carriers, further categorized them according to HDGC clinical criteria (HDGC vs non-HDGC), collected phenotypes, classified variants molecularly and according to CDH1 ACMG/AMP guidelines and performed genotype-phenotype analysis. We found 41 families carrying CTNNA1 germline variants encompassing in total 105 probands and relatives. All probands from 13 HDGC families presented DGC and their average age of onset was 40 ± 17 years; 10/13 (77%) HDGC families carried a pathogenic (P) variant. Most probands from 28 non-HDGC families developed unspecified-BC, as well as most of their relatives; 4/28 (14%) carried a P variant, 16/28 (57%) carried a likely pathogenic (LP) variant, 7/28 (25%) carried variants of unknown significance (VUS) and 1/28 (4%) carried a likely benign variant. Regardless of clinical criteria, 97% (32/33) of probands and relatives from P variant-carrier families had DGC/unspecified-GC. In LP variant-carrier families, 82% (28/34) of probands and relatives had unspecified-BC. Only 2/105 individuals had LBC. A cluster of frameshift and nonsense variants was found in CTNNA1 last exon of non-HDGC families and classified as VUS. In conclusion, current available data confirms an association of CTNNA1 P variants with early-onset DGC, but not with LBC. We demonstrate that in ascertained cohorts, CTNNA1 P variants explain <2% of HDGC families and support the use of ACMG/AMP CDH1 specific variant curation guidelines, while no specific guidelines are developed for CTNNA1 variant classification. Moreover, we demonstrated that truncating variants at the CTNNA1 NMD-incompetent last exon have limited deleteriousness, and that CTNNA1 LP variants have lower actionability than CDH1 LP variants. Current knowledge supports considering only CTNNA1 P variants as clinically actionable in HDGC carrying families.