Merkel is "personally convinced of the need" for a tax on financial market transactions, Steffen Seibert told a regular news conference.
The European Commission and several EU states are determined to introduce a tax on financial transactions but the plans have met with strong opposition, notably from Britain with its powerful financial sector.
Chancellor Angela Merkel's centre-right Christian Democratic Union (CDU) and its coalition partners, the Christian Social Union (CSU) and the Free Democrats (FDP), have been negotiating for weeks with the SPD and the ecologist Greens to get the fiscal pact ratified.
"She will bring up the topic, for example, at the four-way meeting with France, Spain and Italy in Rome on June 22," added the spokesman.
However, Seibert was at pains to stress that Germany "is not the only one in Europe that can decide when such a tax would come" but insisted: "We will be campaigning for it hard."
But German Finance Minister Wolfgang Schäuble said Sunday it was unlikely the tax could be put in place before Germany's legislative elections, due in the autumn of 2013.
"A European tax won't come that quickly," Schäuble told German television network ARD in an interview when asked if the controversial tax proposal could be put in motion before the German parliamentary vote.
His remark came after the leader of the main opposition Social Democratic Party (SPD), Sigmar Gabriel, said Thursday that the government and opposition had reached agreement on such a tax.
The issue has been a major obstacle to the German parliament ratifying the budget-balancing European Union fiscal pact.
The German-inspired pact aims to toughen budgetary discipline in the EU. The treaty needs a two-thirds majority to be ratified by parliament, which Merkel's ruling coalition does not have on its own.
In exchange for their support, the SPD and Greens have demanded growth-boosting measures and a financial transaction tax that would at a minimum cover the eurozone.