Tunable spin texture in ferroelectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi mathvariant="normal">B</mml:mi> <mml:msub> <mml:mi mathvariant="normal">i</mml:mi> <mml:mn>6</mml:mn> </mml:msub> <mml:msub> <mml:mi ma

Momentum-dependent spin polarization exists in ferroelectric materials with strong spin-orbit coupling (SOC), which can be controlled by ferroelectric switching, promising for nonvolatile applications in spintronics. In the present work, we investigate the SOC-induced spin texture in orthorhombic $\mathrm{B}{\mathrm{i}}_{6}{\mathrm{O}}_{9}$, a recently discovered ferroelectric material with robust polarization down to the atomic limit. Utilizing first-principles calculations, we reveal the SOC-induced giant band splitting around the \ensuremath{\Gamma} point and notable spin texture, which can be well described by a k\ifmmode\cdot\else\textperiodcentered\fi{}p Hamiltonian with sizable Rashba parameters of ${\ensuremath{\lambda}}_{R}=0.65\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$ and Dresselhaus parameters ${\ensuremath{\lambda}}_{D}=\ensuremath{-}0.23\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$ at the conduction-band minimum. We also find that the SOC parameters can be effectively modulated by strain, resulting from band splitting and band anticrossing collectively, supported by substrate and strain-dependent calculations. The highly tunable spin texture indicates the opportunities for using $\mathrm{B}{\mathrm{i}}_{6}{\mathrm{O}}_{9}$ for the nonvolatile charge-spin conversion useful in spintronics.