In this article, we investigate the stability analysis and controller synthesis problems for a class of stochastic networked control systems under aperiodic denial-of-service (DoS) jamming attacks. First, an observer is constructed to estimate the unmeasurable states, and then a new adaptive event-triggered mechanism on the basis of the observer is proposed to eliminate the adverse effects of DoS attacks and schedule the transmission instants so as to realize a reduction of transmission burden in the network. Under the proposed event-driven communication scheme, an observer-based controller is designed, and a new switched system with time-varying delays is introduced. Conditions for the underlying systems to be mean-square exponentially stable with a weighted <inline-formula><tex-math notation="LaTeX">L_2</tex-math></inline-formula>-gain are established. Also, conditions to co-design the observer, the controller, and the event-triggered scheme are developed. A mass-spring-damper mechanical system is used to demonstrate the effectiveness and advantages of the new design techniques.