You can test if it actually is in the Python PATH, if you open a cmd and type in chromedriver and hit Enter. If Starting ChromeDriver 2.15.322448 is appearing, the PATH is set appropriately and there is something else going wrong. Then try the following:
Download ChromeDriver
Then you have multiple options:
Add it to your system path
Put it in the same directory as your python script
Specify the location directly via executable_path
driver = webdriver.Chrome(executable_path='C:/path/to/chromedriver.exe')
Before you add the chromedriver to your path, make sure it's the same version as your browser.
If not, you will need to match versions: either update/downgrade you chrome, and upgrade/downgrade your webdriver.