With wireless sensor networks (WSNs) reaching maturity, security is becoming a greater concern. Just like traditional networks benefit from SSL or IPSec, WSNs require a security architecture that prevents basic attacks to data privacy, authenticity and integrity. There are lots of disparate solutions for these problems but nothing to actually put them together into a robust architecture. This paper presents MArSSeNs, a modular architecture for the security of WSNs, which can provide transparent security individually to all the data streams and network layers of an application. It does not require any modification of the application logic but only to its structure. It facilitates security configuration, verification (through code reuse) and reduces the probability of code errors. The tests compare MArSSeNs, an unsecured application and one that implements the same security services but hard-coded. The results show minor overhead in processing time and power consumption. © 2011 IEEE.