He set up a security agency, bringing together counter-terrorism and domestic intelligence under the authority of a single body.
The king also replaced the head of the royal guard, and made several appointments to the team of his son, Crown Prince Mohammed bin Salman.
In June, the 31-year-old prince became the first in line to the Saudi throne.
He replaced the king's nephew, Mohammed bin Nayef, 57, who pledged allegiance to the younger cousin.
The latest changes were announced in Thursday's decrees, published by the state-run SPA news agency.
It said the decisions were made in order to maintain "the highest readiness levels to keep up with the latest developments".
The new security agency is now headed by Abdulaziz bin Mohammed al-Howairini, a general who will be reporting directly to the 81-year-old king.
Previously, the interior ministry was in charge of counter-terrorism and domestic intelligence.
The new boss of the royal guard is Gen Suheil al-Mutiri, replacing Gen Hamad al-Awhaly.
As part of the overhaul, King Salman also named Mohammed el-Kuwaiz as head of the agency that regulates investments in the oil-rich country.
Saudi Arabia's King Salman has issued a series of royal decrees, marking a new shake-up of top officials.