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One of the most frequent sitting styles of Asians in everyday life is a cross-legged sitting. The cross-legged sitting results in higher compression load in spine than sitting on a chair, so a proper sitting posture is more needed. The purpose of this study was to classify the spinal posture during cross-legged sitting from the seat pressure pattern for future usage in the posture monitoring system. Twenty young men participated in this study. The seat pressure was measured for three spinal postures of flat, slump, and lordosis when subjects were instructed to pose a certain posture while seated on the floor with legs crossed. The contact area was divided into feet and buttocks by using a filter with a pressure threshold (${\text{th}}_f$). A decision tree was developed for the classification of three postures, with a decision variable of feet to buttocks pressure ratio. The three spinal postures were classified by comparison of feet-buttocks ratio ($R_{fb})$ and thresholds (${\text{th}}_{R1}$, ${\text{th}}_{R2}$): a slump posture with a greater $R_{fb}$ than ${\text{th}}_{R1}$, a lordosis posture with a smaller $R_{fb}$ than ${\text{th}}_{R2}$. Each threshold was calculated by adding or subtracting a certain percentage ($\alpha ,\beta$) to or from the $R_{fb}$ of flat posture and the classification accuracy was investigated with a range of thresholds. The accuracy of classification achieved 99.38% for certain ranges of thresholds. The developed algorithm showed the best performance when $\alpha$ and $\beta$ were in the range of 2.85–5.67% and 1.58–2.20%, respectively. The feet-buttocks pressure ratio showed significant correlation with lumbar angle ($r=0.67$, $p<0.001$). Anterior and posterior tilts of upper body in the slump and lordosis postures would result in more pressure concentration in the feet and buttocks, respectively, which was incorporated in the classification algorithm of this study. The result of this study could be extended to the real-time or offline monitoring of the sitting posture.