If you are a natural or legal person, engaged in economic activity, and conduct your activity on a continuous and regular basis with the aim of generating income, then you are subject to the regulations and legislation of Value Added Tax (VAT) in Saudi Arabia. There are two types of registration: mandatory and optional.
Optional Registration:
The legislation allows individuals or entities whose supplies or expenses in the past twelve months or expected in the next twelve months exceed 375,000 Saudi Riyals, with an increase of more than 187,500 Saudi Riyals, to optionally register for VAT purposes. Also, those whose supplies consist entirely of zero-rated items are allowed to register optionally. The registration becomes effective from the date of acceptance of the registration application by the authority.
Mandatory Registration:
Registration becomes mandatory for anyone who supplies goods and services subject to VAT, provided that their supplies of taxable goods or services during the past twelve months or expected in the next twelve months reach the mandatory registration threshold of 375,000 Saudi Riyals. Registration must be done within 30 days from the end of the month in which the supplies reach the mandatory threshold, and the registration becomes effective from the beginning of the first month in which the supplies are expected to exceed the mandatory registration threshold.

Tax periods depend on the annual supplies of the taxpayer. The tax period for individuals subject to Value Added Tax (VAT) whose annual taxable supplies exceed 40 million Saudi Riyals over the preceding twelve months is monthly, while for others, it is quarterly every three months.
A taxpayer whose annual supplies do not exceed 40 million Saudi Riyals may apply to the authority to use a monthly tax period, and the authority may approve or reject it.
Similarly, a person who has used the monthly period for two years may apply to the authority to use a quarterly tax period, provided that the value of their supplies during the year does not exceed 40 million Saudi Riyals at the time of application.
A person using the monthly period submits their first return to the authority at the end of the month following the month of supply. For example, they would submit the tax return for January on February 28.
As for a person using the quarterly period, they submit their first return to the authority on the last day of the month following the three-month period of supply. For example, they would submit the tax return for the first quarter of the year on April 30.

Many establishments resident in Saudi Arabia often seek services from companies or individuals outside the Kingdom, whether these services are consultancy, technical, or engineering. When these services are requested from non-residents, they may be subject to withholding tax by the Zakat, Tax, and Customs Authority.
Withholding tax, also known as retention tax, is a direct tax determined at certain rates and applied to non-residents in Saudi Arabia. It is deducted from amounts they receive from a source within the Kingdom. Withholding tax rates in Saudi Arabia range between 5 and 20 percent.
Who is responsible for withholding tax?
The person responsible is defined as any individual residing in Saudi Arabia, whether taxable or non-taxable, or a permanent establishment for non-residents who pays amounts from sources within the Kingdom to non-residents. Thus, they are responsible for remitting the withholding tax amount to the authority.
As for the resident natural person, they are only obligated to withhold tax in cases where they pay an amount from a source within the Kingdom for services provided by a non-resident for the purposes of their economic activity.