County secondary schools in Kenya form the third tier after national and extra-county schools respectively.
Most of Kenyan learners are absorbed by these schools.
Admission to county schools is done online by the Ministry of Education.
They are either mixed or single-sex.
Here is a list of county secondary schools in Isiolo County:
| KNEC CODE | NAME | TYPE | CATEGORY | |
| 1 | 17306301 | OLDONYIRO SECONDARY SCHOOL | Mixed | County |
| 2 | 17322301 | SERICHO SECONDARY SCHOOL | Mixed | County |
| 3 | 17356101 | MERTI SECONDARY SCHOOL | Boys | County |
| 4 | 17356102 | MERTI MUSLIM GIRLSโ SECONDARY SCHOOL | Girls | County |
| 5 | 17356203 | BULESA SECONDARY SCHOOL | Mixed | County |