Horizontal gene transfer (HGT) is the transmission of genetic material between species by ways other than direct (vertical) inheritance from parents to the offspring. HGT is recognized as a major evolutionary force in prokaryotes as it is involved, for example in acquisition of antibiotic resistance or pathogeny. HGT has long been overlooked and considered insignificant in eukaryotes. However, HGT have also played important roles in the evolutionary history and biology of these species, including in animals (1). For example, HGT have contributed to the colonization of land by plants, in the emergence of plant parasitism in nematodes or in the development of capabilities like resistance to extreme temperatures or desiccation in various animals. Progress in genome sequencing technologies has allowed multiple plant, fungal and animal genomes to be publicly released. Systematic searches for HGT events in the root-knot nematodes or in the bdeloid rotifer, have shown that between 3 and 9 % of protein-coding genes in these species were probably of non-animal origin (2,3). However, in the absence of a user-friendly, rapid, and publicly available tool to detect HGT events in any taxonomic group of interest, we still lack a global view of the prevalence of HGT across a tree of life. Here, we propose Alienness, a taxonomy-aware web application that parses homology search (BLAST, Diamond) results against public libraries to rapidly identify candidate HGT in a genome of interest. Alienness takes as input the result of a BLAST of a whole proteome of interest against any protein library providing taxonomic information on hits. The user defines recipient (e.g. metazoan) and donor (e.g. bacteria, fungi) branches of interest in the NCBI taxonomy. Based on the homology search results, Alienness calculates three scores for rapid detection of HGT events: (i) the Alien Index (AI) metrics as described in (4) to detect a significant gap of magnitude in e-values between candidate donor and recipient taxa. (ii) The HGT Index, based on the same principle but using bit-scores instead of e-values as defined in (5). (iii) The Aggregative Hit Score (AHS) based on a normalized sum of candidate donor and recipient bit scores as defined in (6). Alienness provides a series of useful output and allows taxonomically aware exploration of blast / diamond results. The output generated can also be used for phylogenetic confirmation and exploration via the AvP software (6), available at: https://github.com/GDKO/AvP. We confirmed the accuracy of Alienness on phylogenetically confirmed HGT of non-metazoan origin in plant-parasitic nematodes. Alienness scans whole proteomes to rapidly identify possible HGT in any species of interest and thus fosters exploration of HGT more easily and largely across the tree of life.