This paper investigates the bio-convective behavior of a third-grade non-Newtonian nanofluid over a stretching sheet. While the influence of Newtonian fluid flow based on classical Fourier and Fick’s laws has been widely discussed in previous studies, this work focuses on a novel third-grade nanofluid model incorporating various physical effects. Notably, the classical Fourier law is replaced by the Cattaneo–Christov (CC) theory for both heat and mass fluxes, capturing relaxation phenomena in the presence of bioconvective effects. Heat and mass transport are modeled using the CC framework, and nanoscale mechanisms are described via the Buongiorno nanofluid model. The influences of thermophoresis and Brownian motion are analyzed alongside dissipative and radiative effects. The Optimal Homotopy Asymptotic Method (OHAM) is employed to solve the resulting nonlinear equations. Graphical representations of key parameters are presented. Results reveal that the velocity profile increases with higher values of material parameters but decreases with an increase in the Reynolds number. The temperature decreases with higher Prandtl number but increases with greater radiation parameter. The concentration profile is found to decline with increasing Schmidt number.