在计算复杂性理论内，机率图灵机是一个非决定型图灵机，在每个转折点根据某种概率分布随机选择某种可行的转变(transition)。

在转变是均匀分布机率的例子里面，我们可以定义为决定型图灵机多了一个添加的"写入"指令，这一个写入指令的值是所有图灵机能用符号的均匀分布机率选择出的符号 (概括地说，这个写入指令以相同的机率在纸带上面写入'1'或者'0'。) 另一个常用的定义是多了一条随机纸带，上面布满了许多随机比特值的确定型图灵机。

所以，机率图灵机可以有随机的结果(与决定型图灵机不同)；给定一个输入和一个状态机，机器运作的时间长度会不同，或者甚至不会停止； 甚至，这机器可能在这一次操作下回传为接受，下一次相同的输入值却回传为拒绝。

In computability theory, a probabilistic Turing machine is a non-deterministic Turing machine which randomly chooses between the available transitions at each point according to some probability distribution.

In the case of equal probabilities for the transitions, it can be defined as a deterministic Turing machine having an additional "write" instruction where the value of the write is uniformly distributed in the Turing Machine's alphabet (generally, an equal likelihood of writing a '1' or a '0' on to the tape.) Another common reformulation is simply a deterministic Turing machine with an added tape full of random bits called the random tape.