The coupling of the vehicles occurred at 14:07 Beijing time (06:07 GMT; 07:07 BST).
The latest Shenzhou mission was launched on Saturday, taking the nation's first female astronaut into orbit.
Thirty-three-year old Liu Yang flies with Commander Jing Haipeng, 46, and fellow flight engineer, Liu Wang, 42.
It was another two hours after the docking before the astronauts moved between the vehicles.
They first had to wait for pressures inside the vessels to be equalised before they could open the hatches.
Commander Jing led the way into the lab, followed by Mr Liu. Ms Liu initially stayed behind in the Shenzhou capsule in case of emergency.
A video camera inside Tiangong relayed pictures of the entrance of the two men, who waved into the lens. They immediately set about checking systems.
After 20 minutes, Ms Liu followed her male colleagues into Tiangong, and the three then turned to the camera as a group to wave.
This is China's fourth manned mission. It follows on from last year's unmanned Shenzhou-8 outing which completed successful rendezvous and docking manoeuvres at Tiangong.
That gave Beijing authorities the confidence to put astronauts on the current flight.
Monday's docking was an automated procedure; computers - not the crew - were in charge of events.
A suite of radar, laser and optical sensors aligned Shenzhou with Tiangong. The capsule's thrusters then drove it into the space lab's docking ring.
The union happened at an altitude of about 340km (210 miles). Ms Liu operated a handheld video camera to record the moment of docking.
It is understood that for most of the time, only two members of the crew will work in the lab. The third individual will tend to hold back in the Shenzhou craft.
During the flight, a range of scientific experiments are planned, including a number of medical tests geared towards understanding the effects of weightlessness on the human body.
At some point in the next few days, the astronauts will attempt a manual docking.
This would see the crew uncouple their vehicle from the lab, retreat to a defined distance and then command their ship to re-attach itself.
Liu Wang will take the lead in this activity. "We've done over 1,500 simulations," he said during the pre-launch press conference.
"We've mastered the techniques and skills. China has first class technologies and astronauts, and therefore I'm confident we will fulfil the manual rendezvous."