"For example, you might have bought a product from an online retailer -- something most of us have done," he wrote.
"But what the retailer doesn't tell you is that it then turned around and sold or transferred information about your purchase to a 'data broker' -- a company that exists purely to collect your information, package it and sell it to yet another buyer."
Cook noted that tracking this activity is difficult because it remains in a "shadow economy that's largely unchecked" by regulators. He proposed that the Federal Trade Commission require all data brokers to register so you can track where your information gets sold and delete it if you want.
Watch this: Apple, Facebook support more privacy laws
The Apple boss also called on US lawmakers to pass "comprehensive federal privacy legislation," echoing a speech he gave at the European Parliament in November.
The Cupertino, California-based company has been a vocal supporter of consumer privacy in recent years, especially in the wake of Facebook's 2018 Cambridge Analytica data scandal. In October, Apple started letting US users download their data via a privacy portal.