Gleichenella pectinata, known as ‘Star fern’, is a species traditionally used by Amazonian indigenous communities to treat various diseases. The objective of this study was to evaluate the bioactive compounds and antioxidant, antimicrobial, antitumor, and anti-inflammatory activities of G. pectinata leaves. The study included the determination of physicochemical parameters (pH, soluble solids, titratable acidity, moisture, and ash), phytochemical screening, mineral analysis by atomic absorption and quantification of bioactive compounds (vitamin C, organic acids, carotenoids, chlorophylls, and phenols) by liquid chromatography (RRLC). Antioxidant (ABTS and DPPH), antimicrobial (ATCC bacteria and fungi, and multi-resistant strains), antitumor and anti-inflammatory activities were evaluated. The results showed the presence of acetogenins, high concentrations of malic acid (56,559.7 mg/100 g DW), β-carotene (266.6 mg/100 g DW), chlorophyll b (684.7 mg/100 g DW), ferulic acid (3163.5 mg/100 g DW) and quercetin glucoside (945.9 mg/100 g DW). The freeze-dried ethanolic extracts showed greater efficacy against Pseudomonas aeruginosa ATCC (12.0 mg/mL) and multidrug-resistant strains of E. coli (6.6 mg/mL) and P. aeruginosa (6.6 mg/mL). In addition, the extract exhibited moderate antiproliferative activity (IC₅₀: 0.98–1.98 mg/mL) in hepatocellular and cervical carcinoma cell lines. In conclusion, this study provides the first evidence of the antitumor and bioactive potential of G. pectinata, supporting its value as a natural source of functional compounds with potential pharmacological applications.