This is the first of four deck joins, called key segments that will take place this summer.
In total there are five sections of bridge deck and approach roads that need to be joined before the River Mersey is fully bridged, and the deck of the Mersey Gateway Bridge is completely joined up.
The Merseylink construction joint venture of Kier, Samsung C&T and FCC is building the bridge deck in three sections, with the deck being built outwards from each of the three pylons at a rate of around six metres per week.
Hugh O'Connor, general manager of Merseylink, said: "We have now linked the sections of deck together by bridging the gap between the two decks with four steel restraint struts that resemble giant bolts and a concrete pour.
"Once the concrete has set, the decks are further locked together with external post-tensioned tendons. The combination of the concrete, steel struts and external tendons mean the structure can bear hundreds of tonnes of force, helping to hold the bridge in place.
"This is a major step for the project, and we remain on target to complete the final three joins this summer, with the bridge expected to open to traffic this autumn."
Forty cubic metres of concrete were used to fill the two-metre gap between the north approach viaduct and north pylon deck span, in a pour that took around two hours to complete.
"We could only complete the pour once the form traveller was moved into position and the mould had set,” Hugh O'Connor said. “The gap between sections was only two metres wide, but runs the entire 32 metres width of the bridge deck."