iOS Musician Apps (117) Supported Technologies is Inter-App Audio (IAA), Instrument Type is Synth

miRack is a Eurorack-style modular synthesizer. It aims to provide user experience and audio quality close to those of the real Eurorack hardware.

Groove Rider GR-16 is a professional step-sequenced groove box, inspired by professional hardware drum machines and rhythm boxes.

SunVox is a small and fast cross-platform modular synthesizer with pattern based sequencer (tracker).

TweakyBeat is a drum machine synth.

The Moog Model 15 App is the Moog modular synthesizer and synthesis educational tool.

Hexaglyphics is a 13-bit noise generator with an interface based on an original alphabet for hexadecimal values.

Pure Acid is a bassline synthesizer and drum machine in one app.

Moebius Lab is a multimorphic sound processor which contains a synth, a varispeed stereo sample player, effects and LFOs that can modulate each parameter.

SoundScaper is an experimental sound mini lab for creating unusual soundscapes, atmospheric textures, drones, glitches and noises based on circuit bending principles.

Shape is a polyphonic synthesiser allowing the user to draw the shape of the oscillator waveform.

AudioKit Synth One is a completely free & open-source professional iOS Synthesizer. 

iPhilicorda is an emulation of a vintage organ from the sixties, the Philicorda, created by the Dutch company Philips.

PixiTracker 1BIT is a simple and fun tool to quickly create musical sketches, chiptunes and experiments with TRUE 1-BIT SOUND (like on old microcomputers).
No musical knowledge required.

Multilevel Interactive Sound Synthesizer. Up to 240 Frequency Modulation Oscillators or simple Wavetable Additive.

Laplace is a physical-modeling synthesizer based on resonator synthesis that makes it easy to create bowed string, plucked string, blown pipe and metallic sounds.

Jam Synth is a fully featured monophonic guitar synthesizer / multi FX for iOS.