The SI unit of momentum is ________.

Momentum ($p$) is defined as the product of the mass ($m$) and velocity ($v$) of an object:
$$p=m\cdot v$$
The SI unit of mass ($m$) is kilogram ($\text{kg}$).The SI unit of velocity ($v$) is metre per second ($\text{m/s}$).Combining these gives the SI unit of momentum as $\text{kg}\cdot \text{m/s}$, which reads as kilogram-metre per second.