This work presented adsorption characteristics of tetracycline antibiotics (TCs) on KOH-functionalized rice husk biochar pyrolyzed at 700 °C (KBC700) and evaluation on phytotoxicity of TCs-adsorbed aqueous phase to seed germination. Specifically, KBC700 gained eightfold rise in specific surface area by KOH activation. Predominant monolayer chemisorption helped KBC700 control TCs, and spontaneous and exothermic features were identified by thermodynamic studies. KBC700 could efficiently work in a wide pH range (4.5 ~ 9.5), as well as in simulated eutrophic water and co-existing cationic solution. Humic acid exerted negative impact on TCs disposal. Outstanding regeneration capability and stability were also found during adsorption–desorption cycles. Mechanism discussion implied predominant pore filling and π-π interaction accompanied by hydrogen bonding and electrostatic interaction involved in TCs-removal process. Importantly, less phytotoxicity to seed germination was found in TCs-adsorbed aqueous phase. Collectively, these findings contribute to adsorption properties recognition and subsequent application for KOH-modified rice rusk biochar in environmental TCs remediation.