Crimes committed by foreigners dropped 12.7 percent in 2011 to 17,286 cases, a preliminary survey released Thursday by the National Police Agency showed.
The number of foreigners questioned, arrested and handed over to prosecutors last year also fell, totaling 10,061, down 15.2 percent from 2010.
Both numbers have been on the decline since peaking in 2005, the survey said.
Foreigners with permanent residency status were not included in the data.
Among the crimes committed, fake marriage cases soared 26.1 percent to 193 during the period, with the number of foreigners probed rising 17.6 percent to 554.
The police have been clamping down on illegitimate marriages because they are believed to be generating the infrastructure to carry out a host of other criminal activities, the survey said.
Penal Code violations meanwhile slumped by 10.2 percent from the previous year to 12,590, while infringements on the Immigration Control and Refugee Recognition Act and other laws dropped 18.8 percent to 4,696 cases.
By country of origin, China topped the list of law breakers, with 4,012 citizens, accounting for 39.9 percent of the total, followed by regional neighbors South Korea and the Philippines.
The number of foreign suspects who fled overseas in 2011 also slipped, by 4.0 percent, to a total of 677, the NPA's survey said.