Membraneless organelles (MLOs) participate in multiple cellular processes and layers of gene regulation. Alternative polyadenylation (APA) represents a pervasive gene regulatory mechanism that generates distinct 3' untranslated regions (UTRs) in transcripts. However, whether specific MLOs exist in mammals to regulate APA remains largely unknown. Here, we unveil a nuclear body, termed the LENG8 granule (G granule), that functions as an active hub for APA regulation. LENG8 deletion induces extensive 3'UTR lengthening in both cultured cells and male germ cells. Mechanistically, phase separation-driven LENG8 granule formation facilitates LENG8-CPSF4 interaction and promotes the subsequent assembly of the cleavage and polyadenylation (CPA) complex, thereby favoring proximal poly(A) site usage in target transcripts. We further elucidate that YTHDC1-m⁶A landscapes orchestrate LENG8 granule formation/maintenance and target selection. Moreover, LENG8 granules exhibit spatial localization and functional crosstalk with nuclear speckles. Notably, the 3'UTR length of target transcripts correlates with LENG8 granule assembly and properties in a cell-type- and developmental stage-dependent manner. Our study establishes LENG8 as a critical regulator of mRNA 3' end processing and demonstrates that LENG8 granules modulate mammalian APA, advancing our understanding of MLOs-mediated poly(A) site selection.