Some transport services were temporarily suspended, including Gosport Ferry and WightLink services, and Gunwharf Quays was momentarily closed.
Several devices have been found in the harbor since work started in September. However, memories of the bloody conflict still lurk in the 21st century, as evidenced by the discovery of a WWII bomb beneath the waters of Portsmouth harbor.
Dredging is being carried out to deepen and widen a four-mile (7km) channel to allow the the navy's new 65,000-tonne aircraft carriers to dock.
The entrance to Portsmouth Harbour was closed until around 7:30 am as a precaution while the bomb disposal team assessed the swiftest and safest way of removing the device.
These services have now resumed after the harbor reopened just after 0730.
The bomb was unearthed by a dredger early this morning, with the Royal Navy revealing it is a "very serious threat".
The bomb is believed to be a German SC250 weighing 500lbs.
A second World War II bomb has been found in Portsmouth Harbour in just four days, causing trains and ferries to be canceled and shops to be evacuated. It is not known how the device could have ended up at the bottom of the Solent.
The German SC250 bomb is expected to be detonated shortly by Royal Navy bomb disposal experts.
'As always, the Royal Navy stands ready to respond to these call-outs and keep people safe.
"Residents in Portsmouth Harbour and Spice Island and surrounding areas are being notified and offered the relevant safety advice - to stay indoors, open any windows and move away from the outer walls of your house".