For any $N\ge 2$ and $å:=(å_1,\cdots, å_{N+1})\in (0,\infty)^{N+1}$, let $μ^{(N)}_å$ be the corresponding Dirichlet distribution on $\DD:= \big\{ x=(x_i)_{1\le i\le N}\in [0,1]^N:\ \sum_{1\le i\le N} x_i\le 1\big\}.$ We prove the Poincaré inequality $$μ^{(N)}_å(f^2)\le \ff 1 {å_{N+1}} \int_{\DD}\Big\{\Big(1-\sum_{1\le i\le N} x_i\Big) \sum_{n=1}^N x_n(\pp_n f)^2\Big\}μ^{(N)}_å(\d x)+μ^{(N)}_å(f)^2,\ f\in C^1(\DD)$$ and show that the constant $\ff 1 {å_{N+1}}$ is sharp. Consequently, the associated diffusion process on $\DD$ converges to $μ^{(N)}_å$ in $L^2(μ^{(N)}_å)$ at the exponentially rate $å_{N+1}$. The whole spectrum of the generator is also characterized. Moreover, the sharp Poincaré inequality is extended to the infinite-dimensional setting, and the spectral gap of the corresponding discrete model is derived.