VSBW, a joint venture between VolkerStevin and Boskalis Westminster, began work in September on Portsmouth City Council’s £100m Southsea coastal scheme.
Within days the crew had unearthed the remains of old sea defences under Southsea's Clarence Pier Playing Field, during topsoil stripping for a compound area on the field.
Alex Godden, principal consultant for Wessex Archaeology, said: "The remains are highly likely to be those of the wider historic defences, constructed as part of the 17th century remodelling of Portsmouth’s earlier Tudor sea defences.
"Research and analysis of historic mapping suggests that these remains are part of the outer moat wall, which was infilled around 1870 and then buried.
"This outer moat formed part of the fortifications, and the above-ground remains can be seen at the Long Curtain, King's Bastion and Spur Redoubt Scheduled Monument.
"It's an exciting find as it was widely thought that these defences had been demolished, but now we have the opportunity to get insights into military engineering and design technologies dating back to the 17th century onwards."
Councillor Steve Pitt, deputy leader of Portsmouth City Council, said: "These finds are remarkably well preserved – and potentially nationally important – remains of Portsmouth's historic defences that are directly related to the city’s continuing naval history and worldwide significance as a strategic port. The discovery will allow archaeologists to confirm aspects of the defence designs which, until now, have only ever been seen on historic plans."
Historic England has been informed about the discovery and agreed to the mitigation measures that have been adopted.
Southsea coastal scheme project director Guy Mason said: "In accordance with the agreed mitigation measures, we have now covered up these remains, preserving them in situ for possible future examination.
"We are committed to working closely with on-site archaeologists to ensure that any archaeological remains are identified and recorded appropriately, as well as allowing our construction programme to continue without significant delays."
Portsmouth City Council, Wessex Archaeology and Historic England archaeologists maintain regular contact to agree appropriate recording strategies as remains are uncovered. This is in line with the Southsea Coastal Scheme's Historic Environment Mitigation Strategy, as well as planning conditions and Scheduled Monument Consent.