Southwest Border Security Threat Assessment Act of 2017 This bill directs the Department of Homeland Security (DHS) to submit a southwest border threat analysis that includes an assessment of: terrorism and criminal threats posed by individuals and organized groups seeking to unlawfully enter the United States through the southwest border or seeking to exploit security vulnerabilities along such border; improvements needed at and between ports of entry to prevent terrorists and instruments of terror from entering the United States; gaps in law, policy, and coordination that hinder effective and efficient border security, counterterrorism, anti-human smuggling and trafficking efforts; the flow of legitimate trade along the southwest border; the current percentage of situational awareness and of operational control achieved by DHS along the southwest border; the impact of trusted traveler programs on border wait times and border security; and traveler crossing times and any potential security vulnerability associated with prolonged wait times. The bill requires the Border Patrol, within 180 days after submission of the threat analysis and every five years thereafter, to issue a Border Patrol Strategic Plan that includes consideration of such analysis and other specified factors, including: efforts to increase situational awareness, to detect and prevent terrorists and instruments of terrorism from entering the United States, and to detect, interdict, and disrupt aliens and illicit drugs at the earliest possible point upon entry into the United States; staffing requirements; and assessments of training programs for detecting fraudulent documents and of how border security operations affect crossing times.