Can cockroaches come up through drains?

Yes, some cockroaches can enter through drains, particularly where a drain trap is dry or plumbing is damaged. Sewer-associated species such as American cockroaches can use these routes. However, a cockroach beside a sink does not prove it came through the drain; nearby pipe gaps and cupboards can also provide access or shelter. (UC IPM: Cockroach identification and management; UF/IFAS: Cockroach entry and prevention)

Keep water traps in infrequently used fixtures from drying out, check for plumbing faults and inspect gaps around pipes. Repairing the entry route matters more than repeatedly treating the visible drain opening. If sightings continue, have the plumbing and surrounding hiding places checked rather than assuming every bathroom cockroach has the same source. (UF/IFAS: Cockroach entry and prevention)