Background: The tumor suppressor gene ARID1A is an emerging target for new cancer treatment strategies including ATR inhibition. This study aimed to describe the landscape of ARID1A alterations (ARID1Amut) in gynecologic malignancies. Methods: Patients (pts) with a diagnosis of ovarian (OC) or uterine cancer (UC) of different histologies and comprehensive genomic profiling (CGP) by Foundation Medicine Inc. were included in this study. CGP of solid tissue biopsies (Tbx; FoundationOneCDx) included evaluation of genomic loss of heterozygosity (gLOH; gLOH-high as ≥16% as validated in OC), microsatellite instability (MSI), and tumor mutational burden (TMB; high as ≥10 mutations/Megabase). CGP of peripheral whole-blood liquid biopsies (Lbx; FoundationOneLiquidCDx) included evaluation of MSI and tumor fraction (TF), a measure of the relative quantity of circulating tumor DNA (ctDNA). TF was calculated by measures of aneuploidy and allele frequency and binned as TF < 1%, TF 1 to < 10%, or TF ≥10%. Results: Tbx Cohort: 5,778/30,212 (19.1%) cases were ARID1Amut. Pts had a median age of 63 (range 21-89) years. ARID1Amut were observed across many subtypes and most frequently in endometrial endometrioid (n = 3,052, 57.7%) and ovarian clear cell (n = 982, 57.6%) but rarely in serous (OC, n = 12,258, 2.8%; UC, n = 2,682, 8.9%). Pts frequently harbored more than one ARID1Amut (2,360/5,778, 40.8%). Of the 8,484 observed ARID1Amut, 97.6% were short variants (SV), 0.5% were deletions, and 1.9% were inactivating rearrangements. SV were primarily frameshifts (68.5%) and nonsense mutations (27.6%). The most frequent alterations observed were frameshifts at the D1860 codon. SV were predicted to be homozygous in 11.9%, heterozygous in 65.3%, or unknown zygosity in 22.8%. Overall, 16.6% of ARID1Amut cases with SV had at least one homozygous alteration. 6.6% of pts with homozygous ARID1Amut were MSI-high (MSI-H), while 39.4% of pts with only heterozygous or unknown zygosity ARID1Amut were MSI-H (p < 0.0001). Overall, 1,905 (33.0%) of ARID1Amut cases were MSI-H, and 2,183 (37.8%) were TMB high. For ARID1Amut cases with evaluable gLOH (n = 4745), the median gLOH was 2.7%, and 5.9% pts were gLOH-high. The most frequently altered co-occurring genes were PTEN (62.2%), PIK3CA (54.2%), and TP53 (27.6%). 8.7% of ARID1Amut also harbored a BRCA alteration. Lbx Cohort: 59/967 (6.1%) cases were ARID1Amut. 17 (28.8%) were MSI-H. Frequency of ARID1Amut increased as TF increased, with a detected frequency of 1.3%, 6.7%, and 14.0% among Lbx with TF < 1%, TF 1 to < 10%, or TF ≥10% respectively. Conclusions:ARID1Amut are observed across a variety of Gynecologial cancer subtypes but are enriched in clear cell and endometrioid histologies and detected in both tissue and liquid biopsies. ARID1Amut were not associated with elevated gLOH but were often MSI-H and TMB ≥10mut/Mb. Clinical trials targeting ARID1A may wish to consider the context of co-occuring biomarkers.